Safeguard for shift control for auxiliary gearing of automatic transmission

ABSTRACT

In an automatic transmission including an auxiliary gearing drivingly connected to a main gearing, the auxiliary gearing is shiftable between a high gear position wherein a direct clutch is engaged and a reduction brake is disengaged and a low gear position wherein the direct clutch is disengaged and the reduction brake is engaged. A one-way clutch is arranged to complement the action of the reduction brake. A shift valve which controls the auxiliary gearing is subject to a solenoid hydraulic pressure generated by an electro-hydraulic transducer including a solenoid. A reverse-select hydraulic pressure, generated by a manual valve when the manual valve is placed at reverse drive position, is directly applied to the shift valve to hold the shift valve at a low gear position wherein the direct clutch is disengaged and the reduction brake is engaged to condition the auxiliary gearing in the low gear position thereof.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic shift control system for anauxiliary gearing of an automatic transmission, and more particularly toa safeguard of such a shift control.

JP 59-113351 A discloses an automatic transmission including anauxiliary gearing drivingly connected in series to a main gearing.

In this known automatic transmission, the auxiliary gearing is shiftablebetween a high gear position and a low gear position. It includes adirect clutch as a first frictional device to be engaged in the highgear position, and a reduction brake as a second frictional device to beengaged in the low gear position. The high gear position is establishedin the auxiliary gearing when the direct clutch is engaged with thereduction brake released, while the low gear position is established inthe auxiliary gearing when the direct clutch is released with thereduction brake engaged. The auxiliary gearing provides a direct drivein the high gear position, while it provides a reduction gear in the lowgear position. The auxiliary gearing also includes a one-way clutchwhich becomes operative to serve as a substitute for the reduction brakeduring engine drive forward running. During a shift in the auxiliarygearing from the low to high gear position by disengaging or releasingthe reduction brake and engaging the direct clutch, the one-way clutchanchors a rotary member which is to be gripped by the direct clutch.This prevents excessive wear of the direct clutch and reduces a shocktaking place during engagement of the direct clutch.

For reverse drive, the main gearing is conditioned in a reverse gearposition, while the auxiliary gearing is conditioned in the low gearposition since if the auxiliary gearing is conditioned in the high gearposition with the direct clutch engaged and the reduction brakereleased, the auxiliary gearing which tends to rotate as a unit owing tothe engagement of the direct clutch is held stationary by the one-wayclutch complementary to the reduction brake. Thus, in order to protectthe automatic transmission against damage, the auxiliary gearing must beconditioned in the low gear position with the reduction brake engagedand the direct clutch released.

According to JP 59-113351 A, the auxiliary gearing is shiftable betweenthe high gear position and low gear position by a 3-4 shift valve. The3-4 shift valve has a high gear position wherein a servo activatinghydraulic pressure generated by a regulator valve is supplied to thedirect clutch, namely the first frictional device, to engage same andthe reduction brake, namely the second fictional device, is drained. Italso has a low gear position wherein the direct clutch is disengaged andthe reduction brake is engaged. The 3-4 shift valve is shiftable betweenthe high and low gear positions in response to a solenoid hydraulicpressure generated by an electro-hydraulic transducer, a spring and ahydraulic pressure signal supplied thereto via a predetermined hydraulicfluid line. The electro-hydraulic transducer is of the on drain typewhich generates the solenoid hydraulic pressure when its solenoid isdeenergized. The solenoid hydraulic pressure urges the shift valvetoward the high gear position thereof against the action of the spring.A 2-3 shift valve operatively associated with another electro-hydraulictransducer is provided. This electro-hydraulic transducer is of the ondrain type which generates a solenoid hydraulic pressure when itssolenoid is deenergized. This solenoid hydraulic pressure urges the 2-3shift solenoid against a spring toward a position wherein thepredetermined fluid line is drained when the automatic transmission hasa forward drive range. In response to energization of the solenoid ofthe electro-hydraulic transducer for the 2-3 shift valve, theabove-mentioned fluid line is allowed to communicate with the pressureregulator to receive the servo activating hydraulic pressure when theautomatic transmission is conditioned in the first or second gearposition during this drive range or when the automatic transmission isconditioned for a reverse drive range. The predetermined fluid line isdrained when the automatic transmission is conditioned in the third orfourth (overdrive) gear position. Upon presence of the hydraulicpressure signal in this fluid line, the 3-4 shift valve is held orlocked at the low gear position. Thus, the 3-4 shift valve is held at alow gear position thereof owing to the presence of the hydraulicpressure in the fluid line leading from the 2-3 shift valve duringforward running with the first and second gear position during theforward drive range or during reverse drive range. If the solenoids ofboth of these electro-hydraulic transducers are inadvertentlydeenergized when the automatic transmission has the reverse drive range,the 2-3 shift valve is urged against the spring to the above-mentionedposition wherein the predetermined fluid line is drained, allowing the3-4 shift valve to move to the high gear position thereof owing to thesolenoid hydraulic pressure. As a result, the auxiliary gearing tendingto rotate as a unit is anchored by the one-way clutch, and it isinterlocked.

An object of the present invention is to provide an automatictransmission including an auxiliary gearing which has a shift controlfor the auxiliary gearing free from the above-mentioned problem.

A specific object of the present invention is to provide a shift controlfor the auxiliary gearing of the automatic transmission which will holdthe auxiliary gearing in the low gear position thereof when theautomatic transmission has a reverse drive range (R range).

Another and further specific object of the present invention is toprovide a shift control whereby the auxiliary gearing is shifted to thehigh gear position thereof when the automatic transmission has a forwarddrive range if the associated solenoid or solenoids are inadvertentlydeenergized.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an automatictransmission for a motor vehicle with an engine, the automatictransmission having a forward drive range and a reverse drive range, theautomatic transmission comprising: a main gearing; an auxiliary gearingdrivingly connected to said main gearing, said auxiliary gearing havinga high gear position wherein a first frictional device is engaged and asecond frictional device is disengaged and a low gear position whereinsaid first frictional device is disengaged and said second frictionaldevice is engaged, said auxiliary gearing being shiftable between saidhigh gear position and said low gear position, said auxiliary gearingincluding a one-way clutch means for complementing an action of saidsecond frictional device;

means for generating a servo activating hydraulic pressure;

electro-hydraulic transducer means for generating a solenoid hydraulicpressure;

a shift valve fluidly disposed between said hydraulic pressuregenerating means, said first frictional device, and said secondfrictional device, said shift valve being shiftable in response to saidsolenoid hydraulic pressure signal to a high gear position wherein saidservo activating hydraulic pressure is supplied to said first frictionaldevice and said second frictional device is drained, and to a low gearposition wherein said first frictional device is drained and said servoactivating hydraulic pressure is supplied to said second frictionaldevice;

means for acting on said shift valve in such a manner that, when saidelectro-hydraulic transducer means normally operate, said shift valveshifts to said high and low gear positions selectively in accordancewith a predetermined schedule when the automatic transmission has theforward drive range; and

means for holding said shift valve at said low gear position thereofwhen the automatic transmission has the reverse drive range, wherebysaid shift valve is held in said low gear position insuringdisengagement of said first frictional device if said electro-hydraulictransducer means fails to operate normally.

According to one specific aspect of the present invention, theelectro-hydraulic transducer means includes a solenoid and is of the offdrain type that fails to generate the solenoid hydraulic pressure whenthe solenoid is deenergized. The solenoid hydraulic pressure urges theshift valve toward the low gear position thereof, and the acting meansincludes spring means for biasing the shift valve toward the low gearposition thereof, and means for generating a pilot hydraulic pressureacting on the shift valve to urge the shift valve toward the high gearposition thereof against the spring means, whereby if the solenoid ofthe electro-hydraulic transducer means is inadvertently deenergized whenthe automatic transmission has the forward drive range, the acting meansfunctions to hold said shift valve at said high gear position thereof.

According to another specific aspect of the present invention, theelectro-hydraulic transducer means includes a solenoid and is of the offdrain type that fails to generate the solenoid hydraulic pressure whenthe solenoid is deenergized. The solenoid hydraulic pressure urges theshift valve toward the low gear position thereof, and the acting meansincludes spring means for biasing the shift valve toward the high gearposition thereof, whereby if the solenoid of the electro-hydraulictransducer means is inadvertently deenergized when the automatictransmission has the forward drive range, the acting means functions tohold the shift valve at the high gear position thereof.

According to still another specific aspect of the present invention, theelectro-hydraulic transducer means includes a solenoid and is of the ondrain type that generates the solenoid hydraulic pressure when thesolenoid is deenergized. The solenoid hydraulic pressure urges the shiftvalve toward the high gear position thereof, and the acting meansincludes spring means for biasing the shift valve toward said low gearposition thereof, whereby if the solenoid of the electro-hydraulictransducer means is inadvertently deenergized when the automatictransmission has the forward drive range, the acting means functions tohold the shift valve at the high gear position thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B, when combined side by side, are a hydraulic circuitillustrating a hydraulic control system for an automatic transmissionemploying a first embodiment of a shift control according to the presentinvention;

FIG. 2 is a schematic view of a main gearing and an auxiliary gearing ofthe automatic transmission;

FIG. 2A is a table illustrating a shift schedule;

FIG. 3 is a time chart illustrating a build-up of a hydraulic pressuresupplied to a reduction brake RD/B during engine drive forward running(see fully drawn line P_(D)) and that during engine brake forwardrunning (see two-dot chain line P_(E));

FIG. 4 is a diagram showing a second embodiment of a shift controlaccording to the present invention;

FIG. 5 is a diagram showing a third embodiment of a shift controlaccording to the present invention; and

FIG. 6 is a diagram showing a fourth embodiment of a shift controlaccording to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the accompanying drawings and more particularly to FIG. 2,the automatic transmission comprises an input shaft 1 and an outputshaft 2 arranged in line with the input shaft 1. It also comprises amain planetary gearing 3 arranged coaxially with the input shaft 1, andan auxiliary planetary gearing 4 arranged coaxially with the outputshaft 2.

The main gearing 3 is the same, in construction, as that of an automatictransmission described on pages I-1 to I-53 of a service manual entitled"NISSAN FULL-RANGE ELECTRICALLY CONTROLLED AUTOMATIC TRANSMISSION OF THERE4R01A TYPE (A261C07)" published by Nissan Motor Co. Ltd., in 1987. Thesame gear train is disclosed in U.S. Pat. No. 4,730,521 issued on Mar.15, 1988 to Hayasaki et al.

As shown in FIG. 2, the main gearing 3 comprises two planetary gearsets, namely a first planetary gear set 5 and a second planetary gearset 6. These planetary gear sets are in the form of a simple planetarygear set. The first planetary gear set includes a first sun gear 5_(S),a first ring gear R_(R), a plurality of pinions, only one being shown at5_(P), meshing with both the sun and ring gears 5_(S) and R_(R), and afirst carrier 5_(C) rotatably supporting the pinions 5_(P). The secondplanetary gear set includes a first sun gear 6_(S), a first ring gear6_(R), a plurality of pinions, only one being shown at 6_(P), meshingwith both the sun and ring gears 6_(S) and 6_(R), and a first carrier6_(C) rotatably supporting the pinions 6_(P).

The sun gear 5_(S) is adapted to be held stationary by a band brake B/B,and it is connectable to the input shaft 1 by a reverse clutch R/C. Thecarrier 5_(C) is connectable to the input shaft 1 by a high clutch H/C,and it is prevented from rotating in a direction opposite to a directionin which the input shaft 1 rotates owing to the action of a low one-wayclutch L/OWC. This carrier 5_(C) is adapted to be held stationary bymeans of a low reverse brake LR/B. The carrier 5_(C) is connectable toan outer race of a forward one-way clutch F/OWC which has its inner raceconnected to the ring gear 6_(R). The ring gear 6_(R) is connectable tothe carrier 5_(C) by means of an overrunning clutch OR/C. The sun gear6_(S) is connected to the input shaft 1.

The auxiliary gearing 4 comprises a third planetary gear set 7 which isin the form of a simple planetary gear set including a third sun gear7_(S), a third ring gear 7_(R), a plurality of pinions, only one beingshown at 7_(P), each meshing with both the sun and ring gears 7_(S) and7_(R). The ring gear 7_(R) is connected to the carrier 6_(C) that servesas an output element of the main gearing 3. The carrier 7_(C) isconnected to the output shaft 2. The ring gear 7_(R) is connectable tothe sun gear 7_(S) by a direct clutch D/C. A reduction one-way clutchRD/B is arranged in parallel to a reduction brake with respect to thesun gear 7_(S). This one-way clutch RD/B prevents the sun gear 7_(S)from rotating in a direction opposite to the direction in which theinput shaft 1 rotates, although it allows the sun gear 7_(S) to rotatein the same direction as the input shaft 1 does. The reduction brakeRD/B is so constructed and arranged as to hold the sun gear 7_(S)stationary.

The automatic transmission has five gear positions or five speeds and asingle reverse gear position or reverse speed. In accordance with aschedule TABLE shown in FIG. 2A, an appropriate one or ones of theclutches and brakes are engaged to establish a desired one of the gearpositions when a driver manipulates a manual valve to place its spool toone of a plurality of forward drive range positions, namely a drive (D)position, three (III) and two (II) engine braking position, and areverse (R) drive position. Upon selecting a neutral (N) position or aparking (P) position, all of the clutches and brakes related to the mainplanetary gear box 3 are released, interrupting transmission of power tothe auxiliary planetary gearing 4.

The operation of the main and auxiliary gearings 3 and 4 is explained.

The main gearing 3 operates as follows:

For establishing a first gear position in the main gearing 3, theforward clutch F/C is engaged. This connects the forward one-way clutchF/OWC in series with the low one-way clutch L/OWC, preventing the ringgear 6_(R) from rotating in the opposite direction to the direction inwhich the input shaft 1 rotates. Since the ring gear 6_(R) serves as areaction member and the sun gear 6_(S) is rotatable with the input shaft1, rotation of the input shaft 1 causes the pinions 6_(P) to turn abouttheir axes and orbit around the sun gear 6_(S) in the same direction asthe direction in which the sun gear 6_(S) rotates, causing the carrier6_(C) to rotate, at a reduced speed, in the same direction as thedirection in which the sun gear 6_(S) rotates. Thus, the first gearposition is established in the main gearing. Assuming now that a gearratio between the sun gear 6_(S) and the ring gear 6_(R) is α₂ (alpha2), the reduction ratio at the first gear position can be expressed as(1+α₂)/α₂. Engine braking is not produced under this condition since theone-way clutches F/OWC and L/OWC allow the carrier 6.sub. C to rotate ata speed higher than the input shaft 1 does.

If engine braking is desired, it is necessary that the overrunningclutch OR/C and low reverse brake LR/B are both engaged as indicated bytriangles in the TABLE shown in FIG. 2A.

In order for an upshift to the second gear position in the main gearing3, the band brake B/B is engaged with the forward clutch F/C keptengaged, holding the sun gear 5_(S) stationary. Since the sun gear 5_(S)serves as a reaction member and the ring gear 6_(R) is still preventedfrom rotating in the opposite direction, the carrier 6_(C) increases itsspeed. With the same speed of rotation of the input shaft 1, a speed atwhich the carrier 6_(C) rotates at the second gear position is higherthan a speed at which it rotates at the first gear position. Assumingnow that a gear ratio between the sun gear 5_(S) and the ring gear 5_(R)is α₁ (alpha 1), a reduction ratio for second gear position can beexpressed as (α₁ ·α₂ +α₁ +α₂)/α₂ (1+α₁).

If engine braking is desired at the second gear position, it isnecessary that the overrunning clutch OR/C is engaged as indicated by atriangle in the TABLE shown in FIG. 2A.

In order for an upshift to a third gear position or direct drive in themain gearing 3, the high clutch H/C is engaged and the band brake B/B isreleased with the forward clutch F/C kept engaged. This causes the ringgear 6_(R) to rotate in unison with the sun gear 6_(S) connected to theinput shaft 1.

If engine braking is desired under this direct drive condition, it isnecessary that the overrunning clutch OR/C is engaged.

In order for an upshift to a fourth gear position (overdrive) in themain gearing 3, the band brake B/B is engaged with the high clutch H/Cand forward clutch F/C kept engaged. Since the carrier 5_(C) isconnected to the input shaft 1 and the sun gear 5_(S) is heldstationary, rotation of the carrier 5_(C) with the input shaft 1 causesthe ring gear R_(R) and thus the carrier 6_(C) to rotate in the samedirection as the direction in which the input shaft 1 rotates. Thereduction ratio for this fourth gear position (overdrive) can beexpressed as 1/(1+α₁). Since the ring gear 6_(R) is allowed to rotatequicker than the carrier 5_(C) rotates owing to the forward one-wayclutch F/OWC, the forward clutch F/C may be kept engaged.

For establishing the reverse drive in the main gearing 3, the reverseclutch R/C and low reverse brake LR/B are engated. Owing to theengagement of the reverse clutch R/C, the sun gear 5_(S) rotates inunison with the input shaft 1. Since the carrier 5_(C) is heldstationary owing to the action of the low reverse brake LR/B, rotationof the sun gear 5_(S) causes the ring gear 5_(R) and thus the carrier6_(C) to rotate in the opposite direction to the direction in which theinput shaft 1 rotates. The reduction ratio for the reverse drive can beexpressed as -1/α₁.

The operation of the auxiliary gearing 4 is explained.

With the reduction brake RD/B engaged to hold the sun gear 7_(S)stationary, the auxiliary gearing 4 is conditioned in a low gearposition (or a reduction gear). In the low gear position, rotation ofthe carrier 6_(C) transmitted to the ring gear 7_(R) causes the pinions7_(P) to turn about their axes to orbit around the sun gear 7_(S),causing the carrier 7_(C) and thus the output shaft 2 to rotate at areduced speed. Thus, the reduction brake RD/B function as a frictionaldevice for establishing the low gear position in the auxiliary gearing4. A reduction ratio within the auxiliary gearing 4 can be expressed as1+α₃ if a gear ratio between the sun gear 7_(S) and the ring gear 7_(R)is α₃ (alpha 3).

With the direct clutch D/C engaged with the reduction brake RD/Breleased, the sun gear 7S is connected to the ring gear 7_(R), and theauxiliary gearing 4 is conditioned in a high gear position (or a directdrive). In the high gear position, the rotation of the carrier 6_(C)causes the output shaft 2 to rotate at the same speed. Thus, the directclutch D/C serves as a frictional device for establishing the high gearposition in the auxiliary gearing 4.

During a shift from engagement state of the reduction brake RD/B todisengagement state thereof, if the sun gear 7_(S) starts rotating inthe opposite direction prior to engagement of the direct clutch D/C, thedirect clutch D/C wears at a quick rate. Besides, a substantially greatshock takes place during engagement of the direct clutch D/C. In orderto solve this problem, the one-way clutch RD/OWC is arranged to preventsuch undesired rotation of the sun gear 7_(S).

Since the one-way clutch RD/OWC serves the same function as thereduction brake RD/B does in a certain circumstance, it is not necessaryto engage the reduction brake RD/B in such circumstance. However, forless complicated hydraulic control system, the reduction brake RD/B isleft engaged in the above-mentioned circumstance. This results in twooperation modes only, namely, a first mode where the direct clutch D/Cis engaged and the reduction brake RD/B released, a second mode wherethe reduction brake RD/B is engaged and the direct clutch D/C released.

Describing gear positions established in the overall automatictransmission including the main and auxiliary gearings 3 and 4, thefirst gear position (ultra low) is established when the main gearing 3is conditioned in the first gear position and the auxiliary gearing 4 isconditioned in the low gear position. A reduction ratio for the firstgear position can be expressed as (1+α₂)(1+α₃)/α₂. with the auxiliarygearing 4 kept as it is, if the main gearing 3 is shifted to the secondgear position thereof and then to the third gear position (direct drive)thereof, the overall automatic transmission shifts to a second gearposition and then to a third gear position. A reduction ratio for thesecond gear position can be expressed as (α₁ ·α₂ +α₁ +α₂)(1+α₃)/α₂ (α₁+1). A reduction ratio for the third gear position can be expressed as1+α₃. With the main gearing 3 kept in the third gear position (directdrive), if the auxiliary gearing 4 is shifted to the high gear position(direct drive), the overall automatic transmission is shifted to thefourth gear position (direct drive). With the auxiliary gearing 4 keptin the high gear position, if the main gearing 3 is shifted to thefourth gear position (overdrive), the overall automatic transmission isshifted to the fifth gear position with a reduction ratio expressed as1/(1+α₁ ).

With the auxiliary gearing 4 kept in the low gear position, if the maingearing 3 is conditioned in the reverse drive, the overall automatictransmission is shifted to the reverse drive with a reduction ratioexpressed as -(α₃ +1)/α₁.

In this embodiment, the gear ratios α₁, α₂, α₃ are chosen to beappropriate values, namely, 0.441, 0.560, 0.384, falling in a range from0.4 to 0.6, which range is empirically determined as being suitable forstrength and endurability of the planetary gear sets 5, 6 and 7. As willbe appreciated from the values for the reduction ratios as tabulated inFIG. 2A, the appropriate reduction ratios are provided with asufficiently large span between the first gear position and fifth gearposition.

Referring to FIGS. 1A and 1B, the hydraulic control system is described.The hydraulic control system comprises the following components:

a pump O/P;

a pressure regulator valve 20;

a duty solenoid 24;

a pilot pressure valve 20

a pressure modifier valve 26;

a modifier accumulator 28;

an accumulator control valve 30;

a torque converter relief valve 32;

a lock-up control valve 34;

a lock-up solenoid 36;

a manula valve 38;

a first shift solenoid A;

a second shift solenoid B;

a third shift solenoid C;

an overrunning clutch solenoid 40;

a first shift valve 42;

a second shift valve 44;

a third shift valve 46;

a 5-2 relay valve 48;

a 5-2 sequence valve 50;

a 1-2 accumulator valve 52;

a N-D accumulator 54;

an accumulator 56;

an accumulator shift valve 58;

an accumulator 60;

an overrunning clutch control valve 62;

an overrunning clutch pressure reduction valve 64;

a reduction timing valve 66;

a reduction brake accumulator 68;

a direct clutch accumulator 70; and

a I & II range pressure reduction valve 72.

The components listed as above are connected to a torque converter T/C,forward clutch F/C, high clutch H/C, band brake B/B, reverse clutch R/C,low reverse brake LR/B, overrunning clutch OR/C, direct clutch D/C andreduction brake RD/B, as illustrated in FIGS. 1A and 1B.

The torque converter T/C is of the well-known lock-up type and has arelease chamber REL and an apply chamber APL. Supply of hydraulic fluidto the release chamber REL and discharge thereof from the apply chamberAPL cause the torque converter T/C to assume a torque converter state.On the contrary, supplying the hydraulic fluid to the apply chamber APLand discharging same from the release chamber REL cause the torqueconverter T/C to assume a lock-up state.

The band brake B/B is activated by a servo actuator which issubstantially the same as a servo actuator shown in FIG. 3 of U.S. Pat.No. 4,730,521 issued to Hayasaki et al., on Mar. 15, 1988. The servoactuator for the band brake B/B has a servo apply chamber 2S/A adaptedto be pressurized for establishing the second gear position in theautomatic transmission, a servo release chamber 3,4S/R (corresponding toa servo release chamber 3S/R of the U.S. Pat. No. 4,730,521) adapted tobe pressurized for establishing the third or fourth gear position in theautomatic transmission, and a servo apply chamber 5S/A (corresponding toa servo apply chamber 4S/A of the U.S. Pat. No. 4,730,521) adapted to bepressurized for establishing the fifth gear position in the automatictransmission. Upon application of hydraulic pressure to the servo applychamber 2S/A only, the band brake B/B is engaged. When the servo releasechamber 3,4SR is pressurized with the servo apply chamber 2S/A keptpressurized the band brake B/B is released. When the servo apply chamber5S/A is pressurized with the other two chambers 2S/A and 3,4S/R keptpressurized, the band brake B/B is engaged again.

The pressure regulator valve 20 is formed with a valve bore andcomprises a spool 20b and a plug 20c disposed in the valve bore. Springs20a and 20j are disposed in the valve bore between the spool 20b and theplug 20c to bias the spool 20b to a spring set position thereof asillustrated in FIG. 1A. Viewing in FIG. 1A, the spring 20b has a lowerend bearing against a spring retainer fixed to the valve bore defining awall and an upper end bearing against the spool 20b, while the otherspring 20j has a lower end bearing against the plug 20c and an upper endbearing against the spool 20b. The pressure regulator valve 20 issupplied with hydraulic fluid discharged to a line pressure fluid line81 by the pump O/P and effects a pressure regulation to adjust apressure of the hydraulic fluid in the line pressure fluid line 81 to avalue that is a function of and thus variable with a force with whichthe spool 20b is biased by the springs 20a and 20j. The force with whichthe spring 20j biases the spool 20b is increased in response to amovement of the plug 20c towards the spool 20b. The fluid pressurewithin the line pressure fluid line 81, hereinafter referred to as aline pressure, is increased accordingly. The spool 20b has a pressureacting area 20d which is exposed to the hydraulic fluid downstream of anorifice 82. In accordance with the hydraulic pressure applied to thepressure acting area 12d, the spool 20b is urged downwards as viewed inFIG. 1A against the action of the springs 20a and 20j. The pressureregulator valve 20 has ports 20g, 20f, 20e and 20h formed in the valvebore defining wall within an area within which the spool 20b is adaptedto move. The port 20e is connected to the line pressure fluid line 81,and arranged such that it communicates with the port 20h as the spool20b begins to move downwards and with the port 20f also as the spool 20bmoves downwards further, as viewed in FIG. 1A. The port 20g serves as adrain port. The arrangement among the ports 20g, 20f , and 20e is suchthat the port 20g is gradually covered as the spool 20b moves downwardsfrom the illustrated position in FIG. 1A and it is completely coveredwhen the spool 20b assumes a predetermined position. At thepredetermined position or immediately after the spool 20b has assumedthis predetermined position during this downward movement thereof, theport 20e is about to communicate with the port 20f, and furthersubsequent movement causes the fluid communication between the ports 20eand 20f to increase. The port 20f is connected to a fluid line 84 at oneend thereof. The opposite end of this fluid line 84 is connected to thecapacity control actuator 85. A bleeder 83 is open to the fluid line 84at a portion between the one and the opposite ends. In order to suppresspulsation of the hydraulic fluid pressure within the fluid line 84, afeedback accumulator 86 is connected to the fluid line 81 at a portiondownstream of that portion where the bleeder 83 is arranged.

The pump O/P is a variable volume vane pump which is driven by theengine. An eccentricity within the pump O/P is decreased by the actuator85 when the hydraulic pressure applied to the actuator 85 exceeds acertain value. This causes a reduction in capacity of the pump O/P.

The plug 20c of the pressure regulator valve 20 has a lower end, asviewed in FIG. 1A, serving as a pressure acting area exposed to amodifier pressure supplied to the pressure regulator valve 20 from amodifier pressure fluid line 87. It is formed with another pressureacting area 20i which is exposed to a hydraulic pressure supplied to thepressure regulator valve 20 from a reverse-select fluid line 88. Theplug 20c is urged upwards as viewed in FIG. 1A in response to one ofthese two hydraulic pressures, thus compressing the spring 20j.

With the pressure regulator valve 20 in the illustrated spring setposition, immediately after hydraulic fluid is discharged by the pumpO/P. this hydraulic fluid flows into the fluid line 81. As long as thespool 20b stays in the illustrated spring set position, the hydraulicfluid is trapped, causing the hydraulic pressure in the fluid line 81 toincrease. This pressure is transmitted via the orifice 82 to thepressure acting area 20d, urging the spool 20b downwards against thesprings 20a and 20j, uncovering the port 20h to allow same tocommunicate with the port 20e. Then, the hydraulic fluid is dischargedvia the port 20h, causing a drop in the hydraulic pressure within thefluid line 81. This drop in the hydraulic pressure allows the spool 20bto move back and upwards owing to the action of the springs 20a and 20j.These movements of the spool 20b are repeated until the hydraulicpressure within the fluid line 81 becomes equal to a value that is afunction of the total force with which the springs 20a and 20j bias thespool 20a. The plug 20c is subject to an upward force owing to themodifier pressure from the fluid line 87. Thus, the force with which thespring 20j biases the spool 20b increases in proportion to the modifierpressure in the fluid line 87. Since the modifier pressure is availableduring all drive ranges but the reverse (R) drive range and increases inproportion to an increase in a load on the engine (or an engine outputtorque), the magnitude of the line pressure within the fluid line 81increases in response to an increase in the engine load during all ofthe drive ranges except the reverse (R) drive range.

Upon selecting the reverse drive, the hydraulic pressure as high as theline pressure, is applied to the plug 20c from the fluid line 88, urgingthe plug 20c upward viewing in FIG. 1A. Thus, the line pressure in thefluid line 81 is increased to a value suitable for reverse drive.

When the engine revolution speed exceeds a predetermined value, the pumpO/P driven by the engine discharges hydraulic fluid into the fluid line81 at an excessive rate. Under this condition, the hydraulic pressurewithin the fluid line 81 tends to increase to an excessively high value.This causes the spool 20 to move further downwards to increase fluidcommunication between the ports 20f and 20e, allowing a build-up of afeedback pressure in the fluid line 84. This feedback pressure increasesas the revolution speed of the pump O/P increases, causing the actuator85 to decrease the eccentricity within the pump O/P, thus decreasing thecapacity of the pump O/P. Thus, when the revolution speed of the pumpO/P is higher than a certain value, the flow rate of hydraulic fluiddischarged by the pump O/P is kept substantially constant owing to thecapacity control, preventing a power loss of the engine.

The line pressure within the fluid line 81 is supplied to the pilotvalve 22, the manual valve 38, the third shift valve 46 and theaccumulator 56.

The pilot valve 22 comprises a spool 22b biased by a spring 22a to aspring set position as illustrated in FIG. 1A. The spool 22b has one enddefining a chamber 22c and exposed thereto. The pilot valve 22 isprovided with a drain port 22d, and connected to a pilot pressure line90 with a filter 89. The pilot pressure line 90 is connected via anorifice 91 to the chamber 22c.

FIG. 1A illustrates the pilot valve 22 in the spring set position. Thepilot valve 22 functions to effect a pressure regulation based on thehydraulic fluid supplied thereto from the fluid line 81 to generate apilot pressure in the fluid line 90. The hydraulic fluid supplied fromthe fluid line 81 enters the pilot pressure line 90 and flows via theorifice 91 to the chamber 22c. If a hydraulic pressure within thechamber 22c increases, the spool 22b moves to the right as viewed inFIG. 1A against the spring 22a. When this pressure exceeds a constantvalue that is a function of a force of the spring 22a, the spool 22buncovers the drain port 22d, causing a drop in the pressure in the pilotpressure line 90. Thus, the hydraulic pressure in the pilot pressureline 90 is kept at this constant value. This hydraulic pressure iscalled "a pilot pressure." This pilot pressure is distributed throughthe pilot pressure line 90 to the shift solenoids A, B, C, overrunningclutch solenoid 40, pressure modifier valve 26, orifices 92, 93, lock-upcontrol valve 34, lock-up solenoid 36, and third shift valve 46.

The duty solenoid 24 has a drain line 94 leading from a drain port tothe orifice 92. Normally, i.e., when it is turned OFF or deenergized,the duty solenoid 24 closes the drain port of the drain line 94. When itis turned ON or energized, the duty solenoid 24 opens the drain port ofthe drain line 94. This duty solenoid 24 and the other solenoidsdescribed later are controlled by a computer. The control strategy issuch that as a duty ratio increases, a hydraulic pressure within thedrain line 94 drops. The duty ratio is a ratio of a duration of timewhen a solenoid is turned ON to a predetermined ON-OFF cycle time andexpressed in terms of percentage. Specifically, when the duty ratio is0%, the hydraulic pressure within the drain line 94 is kept as high asthe pilot pressure, while, when the duty ratio is 100%, the hydraulicpressure within the drain line 94 is 0 (zero). The duty ratio is relatedto the magnitude of the engine load (for example, opening degree of theengine throttle valve) such that during all drive ranges but the reverse(R) drive range, the duty ratio is decreased as the magnitude of engineload increases, causing the hydraulic pressure within the drain line 94to increase as the magnitude of engine load increases During reverse (R)drive range, the duty ratio is kept at 100%, thus causing the hydraulicpressure within the drain line 64 to be 0 (zero)

The pressure modifier valve 26 comprises a spool 26b assuming a springset position as illustrated in FIG. 1A owing to a spring 26a and thehydraulic pressure within the drain line 94. The pressure modifier valve26 also comprises an outlet port 26c connected to the pressure modifierline 87, an inlet port 26d connected to the pilot pressure line 90, aport 26g, and a drain port 26e. The spool 26b defines a feedback chamber26f and has an end exposed to same. Axially formed through the spool 26bis an orifice bore 26h having one end communicating with the modifierpressure line 87 and an opposite end communicating with the feedbackchamber 26f. The spool 26b is formed with a pressure acting area exposedto the pilot pressure supplied to the port 26g. A force with which thespring 26a biases the spool 26b is opposed to a force with which thepilot pressure acting via the port 26g on the pressure acting area ofthe spool 26b.

The spool 26b of the pressure modifier valve 26 is subject to the forceowing to the spring 26a and another force owing to the hydraulicpressure within the drain line 94 and urged downwards as viewed in FIG.1A. The spool 26b is subject to a force owing to the modifier pressurewithin the feedback chamber 26f and the force owing to the pilotpressure supplied to the port 26g and urged upwards as viewed in FIG.1A. If the total of the forces urging the spool 26b downwards tends tobe greater than the total of the forces urging the spool 26b upwards,the spool 26b uncovers the inlet port 26d, allowing an increase in thehydraulic pressure within the feedback chamber 26f and the modifierpressure line 87. This increase in the hydraulic pressure in thefeedback chamber 26f increases the total of the forces urging the spool26b upwards. If the total of the forces urging the spool 26b downwardsbecomes less than the total of the forces urging the spool 26b upwards,the spool 26b allows communication of the outlet port 26c with the drainport 26e, causing a drop in the hydraulic pressure in the modifierpressure line 87 and in the feedback chamber 26f. This drop in thehydraulic pressure in the feedback chamber 26f decreases the total ofthe forces urging the spool 26b upwards. Thus, a value taken by themodifier pressure in the line 87 is proportional to a result fromsubtracting the force owing to the pilot pressure supplied to the port26g from the sum of a force of the spring 26a and the force owing to thehydraulic pressure from the drain line 94. Via the modifier pressureline 87, the modifier pressure is applied to the plug 20c. It will nowbe understood from the preceding description that the modifier pressureis proportional to the hydraulic pressure within the drain line 74 buthas a magnitude amplified by the above-mentioned subtracting. Themodifier pressure increases as the engine load increases during alldrive ranges but the reverse (R) drive range since the hydraulicpressure within the drain line 94 increases as the engine load increasesduring all drive ranges but the reverse (R) drive range. During thereverse (R) drive range, the modifier pressure is the minimum valuesince the hydraulic pressure within the drain line 94 is 0 (zero). Sincethe modifier pressure of the above variation characteristic urges theplug 20c in such a direction as to compress the spring 20j, the linepressure within the fluid line 81 increases as the engine load increasesduring all drive ranges but the reverse (R) drive range. In order tosuppress pulsation in the hydraulic fluid within the modifier pressureline 87, the modifier accumulator 28 is provided.

The torque converter relief valve 32 comprises a spool 32b biased by aspring 32a to assume a spring set position as illustrated in FIG. 1A. Inthe illustrated position, the spool 32b allows an outlet port 32c tocommunicate with an inlet port 32d. As the spool 32b moves upwards, asviewed in FIG. 1A, from the illustrated position, the above-mentionedcommunication is decreased and then the outlet port 32c is allowed tocommunicate with a drain port 32e. The spool 32b is formed with anorifice bore 32g to allow a feedback of the output pressure to afeedback chamber 32f. The outlet port 32c communicates via a reliefvalve 95 with a front portion FR/LUB to be lubricated, and it alsocommunicates with a lock-up control valve 34 through a fluid line 96.The inlet port 32d is connected to the port 20h of the pressureregulator valve 20 via a fluid line 97. Via the fluid line 97, thehydraulic fluid discharged by the pressure regulator valve 20 isadmitted to the inlet port 32d and used as a working hydraulic fluid forthe torque converter T/C.

With the torque converter relief valve 32 in the illustrated position,if hydraulic fluid is supplied to the inlet port 32d from the port 20hof the pressure regulator valve 20, it is supplied through the fluidline 96 and the lock-up control valve 34 to the torque converter T/C.Subsequently, upon build-up of pressure in the hydraulic fluid at theoutlet port 32c, this pressure is transmitted via the orifice bore 32gto the feedback chamber 32f, urging the spool 32b to move upwards asviewed in FIG. 1A against the spring 32a. If the pressure exceeds a ofthe spring 32a, the spool 32b allows the outlet port 32c to communicatewith the drain port 32e, causing a drop in the hydraulic pressure at theoutlet port 32c. In this manner, the hydraulic pressure at the outletport 32c is kept at or below the predetermined value. If the hydraulicpressure at the outlet port 32c tends to further increase beyond theabove-mentioned predetermined value, the relief valve 95 opens to permitescape of excessive hydraulic fluid toward the front portion FR/LUB tobe lubricated. This prevents deformation of the torque converter T/C.

The lock-up control valve 34 comprises a spool 34a, a first stepped plug34b on one end of the spool 34a, and a second stepped plug 34c on theopposite end of the spool 34a. A spring 34d is operatively disposedbetween the spool 34a and the plug 34c. The spool 34a is moveablebetween a lock-up or lower limit position as illustrated in FIG. 1A anda lock-up release or upper limit position as viewed in FIG. 1A. When thespool 34a assumes the upper limit position, the fluid line 96 isconnected to a fluid line 98 communicating with the release chamber RELof the torque converter T/C and a fluid line 99 communicating with theapply chamber APL of the torque converter T/C is connected to a drainline 100. Under this condition, the hydraulic fluid from the fluid line96 flows into the torque converter T/C from the release chamber RELtoward the apply chamber APL, rendering the torque converter T/Coperable in the torque converter state thereof. The hydraulic fluidhaving past the torque converter T/C is admitted to the oil cooler COOLvia the drain fluid line 100. The fluid is cooled down at the coolerCOOL and then directed toward a rear portion RR/LUB to be lubricated.When the spool 34a assumes the lower limit position as illustrated inFIG. 1A, the fluid line 96 is connected to the fluid line 99 and thefluid line 98 is connected to a drain port 34e. Under this condition,the hydraulic fluid flows within the torque converter T/C from the applychamber APL toward the release chamber REL, rendering the torqueconverter T/C operable in the lock-up state thereof. In the lock-upstate, the hydraulic fluid having past through the torque converter T/Cis discharged from the drain port 34e and is not directed toward the oilcooler COOL. However, the lubrication of the rear portion RR/LUB isassured by directing the hydraulic fluid from the fluid line 96 to thecooler COOL owing to the provision of orifices 101 and 102.

In order to control the position which the spool 34a assumes, a drainline 103 is connected to a chamber 34f defined between the spool 34a andthe plug 34c. This drain line 103 is connected to the pilot pressureline 90 via an orifice 93, and has a drain port normally closed by alock-up solenoid 36. The hydraulic fluid within the fluid line 98connected to the release chamber REL acts via an orifice 104 on thestepped plug 34b, and the pilot pressure within the pilot pressure line90 acts via an orifice 105 on the stepped plug 34b, urging the steppedplug 34b downwards, as viewed in FIG. 1A.

The lock-up solenoid 36 is turned ON (or energized) or OFF (deenergized)under the control of the computer. It is judged by the computer whetheran operating condition in which the vehicle is involved demands that thetorque converter T/C operate in the lock-up state or not. If it is notdemanded that the torque converter T/C should operate in the lock-upstate, the lock-up solenoid 36 is turned OFF to close the drain port ofthe drain line 103, allowing the pilot pressure to build up in the drainline 103. This pilot pressure is supplied to the chamber 34f to assistthe action of the spring 34d and urges the spool 34a to move against thehydraulic pressures acting on the plug 34b toward the upper limitposition as viewed in FIG. 1A. With the spool 34a in the upper limitposition, the torque converter T/C is rendered to assume the converterstate thereof. When it is judged that the torque converter T/C shouldlock up, the lock-up solenoid 36 is turned ON or energized, opening thedrain port of the drain line 103. No pressure prevails in the drain line103. This allows the spool 34a to assume the lower limit position asillustrated in FIG. 1A since the pilot pressure acting via the orificeon the stepped piston 34b urges the stepped plug 34b and the spool 34downwards against the spring 34d, rendering the torque converter T/C tobe operable in the lock-up state.

In the present embodiment, the torque converter T/C is prevented frombeing operable in the lock-up state thereof during forward running withthe first gear position or during reverse running. In order toaccomplish this object, the chamber 34g which the plug 34c is exposed tois connected via a fluid line 106 to an outlet port of a shuttle valve107 which has two inlet ports connected to the reverse-select fluid line88 and a first-select fluid line 108. When a hydraulic pressure prevailsin one of the fluid lines 88 and 108, this hydraulic pressure istransmitted via the shuttle valve 107 to the fluid line 106 and thus tothe chamber 34g, urging the plug 34c and spool 34a upwards, viewing inFIG. 1A, for movement toward the upper limit position thereof, causingthe torque converter T/C to be operable in the converter state thereof.The pilot pressure coming from the orifice 105 always acts on thestepped plug 34b downwards, thus preventing pulsation of the steppedplug 34b, the spool 34a, and the stepped plug 34a.

The manual valve 38 comprises a spool 38a manually operable by a driverto move among a park (P) position, a reverse (R) position, a neutral (N)position, a forward automatic drive (D) range position, a three (III)range engine braking position, a two (II) range engine braking position.The manual valve 38 is formed with an inlet port connected to the linepressure fluid line 81, a drain port, and outlet ports 38R, 38D, 38III,and 38II. The following TABLE shows which one or ones of the outletports are allowed to communicate with the inlet port connected to theline pressure fluid line 81 in accordance with the various positionswhich the spool 38a is placed at.

    ______________________________________                                        PORT       38R    38D         38III                                                                              38II                                       ______________________________________                                        R          ∘                                                      N                                                                             D                 ∘                                               III               ∘                                                                             ∘                                   II                ∘                                                                             ∘                                                                      ∘                              ______________________________________                                    

The port 38D is connected to a D range pressure fluid line 110 which isconnected to the accumulator control valve 30, the forward clutch F/C,the accumulator shift valve 58, the first shift valve 42, the secondshift valve 44, and the overrunning clutch control valve 62. The port38III is connected to a III-range pressure fluid line 111 which isconnected to the other inlet port of the shuttle valve 112. The port38II is connected to a II-range pressure fluid line 113 which isconnected to the II range pressure reduction valve 72. The port 38R isconnected to the reverse-select pressure fluid line 88. The fluid line88 is connected to the pressure regulator valve 20 and also to one inletport of the shuttle valve 107 to perform the before mentioned boost-upfunction of the line pressure and the lock-up prohibiting function.Besides, the fluid line 88 is connected via an one-way orifice 114 and ashuttle valve 115 to the low reverse brake LR/B, and it is alsoconnected via an one-way orifice 117 to the reverse clutch R/C.

The accumulator control valve 30 comprises a spool 30a with two landshaving different diameters. A lower one of these two lands, as viewed inFIG. 1A has a larger diameter than the other upper land and thus definetherebetween a differential pressure acting area. The land with thelarger diameter is exposed to a chamber 30b which communicates with thefluid line 94, while the land with the less diameter is exposed to adraining chamber 30c. The spool 30a is urged upwards, as viewed in FIG.1A, in response to the hydraulic pressure prevailing in the fluid line94. As before discussed, the hydraulic pressure in the fluid line 94 isadjustable under the control of the duty solenoid 24. When the spool 30ahas moved upwards viewing in FIG. 1A and the outlet port 30d is isolatedfrom a drain port 30e and connected to the D-range pressure fluid line110, an accumulator back-up pressure prevails at a port 30d. Thispressure is applied to the differential pressure acting area definedbetween the two different diameter lands of the spool 30a, urging thespool 30a downwards, as viewed in FIG. 1A, against the fluid pressuresupplied to the chamber 30b. The spool 30a strokes until the accumulatorback-up pressure balances with the pressure within the chamber 30b. Thusthe accumulator back-up pressure at the port 30d is variable in responseto the pressure within the chamber 30b. Since the hydraulic pressurewithin the chamber 30b is variable with the engine load during all driveranges but the reverse drive range owing to the control by the dutysolenoid 24, and the pressure within the D-range pressure line 110prevails during forward drive ranges only, the accumulator back-uppressure prevails during forward drive ranges, only, and increases asthe engine load increases. Thus, the accumulator back-up pressure isengine load responsive. The accumulator back-up pressure is supplied viaan accumulator back-up pressure fluid line 116 to the 1-2 accumulatorvalve 52, N-D accumulator 54, accumulator 60, direct clutch accumulator70, and overrunning clutch pressure reduction valve 64.

Disposed in that portion of the D-range pressure fluid line which allowssupply of hydraulic fluid to the forward clutch F/C is a one-way orifice120. Via a one-way coupling 121, the N-D accumulator 54 and theaccumulator shift valve 58 are fluidly connected in series in this orderto that portion of the D-range pressure fluid line 110 which extendsbetween the one-way orifice 120 and the forward clutch F/C.

The first shift valve 42 comprises a spool 42b biased by a spring 42a toassume a first or spring set position as illustrated in FIG. 1A. Thisspool 42b is urged to move upwards when the pilot pressure within thefluid line 90 is supplied to a chamber 42c when the first shift solenoidA is turned ON to close the associated drain port. When the spool 42b isin the spring set position as illustrated in FIG. 1A, the D-rangepressure line 110 is allowed to communicate with the second gearpressure fluid line 122, the first gear-select pressure fluid line 108is allowed to communicate with a drain port 42d, a II-range pressurefluid line 124 is allowed to communicate with a drain port 42f, andfluid lines 125 and 126 are allowed to communicate with each other. Whenthe spool 42b is urged to move to a second or upper position as viewedin FIG. 1A, the first gear-select pressure fluid line 108 is isolatedfrom the drain port 42d and allowed to communicate with a fluid line127, the fluid line 122 is isolated from the fluid line 110 and allowedto communicate with a fluid line 128, the fluid line 122 is isolatedfrom the D-range pressure line 110 and allowed to communicate with afluid line 128, the fluid line 124 is isolated from the drain port 42fand allowed to communicate with a fluid line 129, and the fluid line 126is isolated from the fluid line 125 and allowed to communicate with thefluid line 128.

The fluid line 122 extends via a check valve 130 to the servo applychamber 2S/A of the band brake B/B. The fluid line 124 is connected tothe corresponding inlet of the shuttle valve 115. The fluid line 125 isconnected to the 5-2 relay valve 48 and also to the overrunning clutchcontrol valve 62. The fluid line 127 is connected on one hand to the 5-2sequence valve 50 and on the other hand to the second shift valve 44.The fluid line 128 extends via one way orifices 131 and 132 to the highclutch H/C and it extends also to the second shift valve 44. The fluidline 129 is connected to the second shift valve 44.

The second gear pressure line 122 is provided with a one-way check valve130. A bypass circuit 133 is connected to the fluid line 122 in parallelto the one-way check valve 130. The 1-2 accumulator valve 52 is disposedin this bypass circuit 133. Thus 1-2 accumulator valve 52 comprises aspool 52a with a small diameter end and a large diameter end, a spring52b acting on the large diameter end of the spool 52a, and a spring 52cacting on the small diameter end of the spool 52a. The spool 52a isformed with a shoulder 52d serving as a pressure acting area. Acting onthis pressure acting area is the accumulator back-up pressure. The spool52a is urged downwards as viewed in FIG. 1A owing to the accumulatorback-up pressure acting on the shoulder 52d and a difference in forceswith which the springs 52b and 52c bias the spool 52a. The spool 52ainitially assumes a position downwards from an equalibrium position asillustrated in FIG. 1A. When the spool 52a is in this initially assumingposition, an outlet port 52e is allowed to communicate with an inletport 52f, permitting the hydraulic fluid to bypass the one-way checkvalve 130 to cause a hydraulic pressure to act within the servo applychamber 2S/A. This hydraulic pressure is transmitted via an orifice 134to a feedback chamber 52g, pushing back the spool 52a upwards as viewedin FIG. 1A. When the hydraulic pressure supplied to the chamber 52gexceeds a predetermined value determined in response to the forcederived from the accumulator back-up pressure acting on the shoulder 52dand the difference in forces with which the springs 52b and 52c bias thespool 52a, the spool 52a allows the outlet port 52e to communicate witha drain port 52h, discharging a portion of the hydraulic fluid to causea drop in the hydraulic pressure. Thus, this hydraulic pressure isadjusted to the above mentioned predetermined value.

The hydraulic pressure appearing at the outlet port 52e and actingwithin the servo apply chamber 2S/A is transmitted via an orifice 136,owing to the provision of a coupling 136 designed to prevent a flow ofhydraulic fluid bypassing this orifice 136, to a piston cap 52i whichthe spring 52c acts on. Thus, the piston cap 52i moves downwards, asviewed in FIG. 1A, against the spring 52c, causing a gradual increase inthe force which the spring 52c acts on the spool 52a. This causes thehydraulic pressure acting within the servo apply chamber 2S/A toincrease at a predetermined rate with respect to time. Since theaccumulator back-up pressure acting on the shoulder 52d of the spool 52aincreases with an increase in engine load, a level assumed by thehydraulic pressure acting within the second gear servo apply chamber2S/A during a period when it increases at the above-mentionedpredetermined rate increases with an increase in engine load.

The second shift valve 44 comprises a spring 44a and a spool 44b biasedby the spring 44a to assume a first or spring set position asillustrated in FIG. 1B. The spool 44b is exposed to a chamber 44c. Whenthe pilot pressure is supplied to this chamber 44c with the second shiftsolenoid B being turned ON to close the associated drain port, the spool44b is urged to move against the spring 44a upwards, as viewed in FIG.1B, from the illustrated spring set position. When the spool 44b assumesthe illustrated position, the fluid line 127 is allowed to communicatewith a drain port 44d, and the fluid line 110 is allowed to communicatewith the fluid line 128, while when the spool 44b moves upwards to asecond or upper position owing to the supply of the pilot pressure tothe chamber 44c, the fluid line 110 is allowed to communicate with thefluid line 127, the fluid line 128 is allowed to communicate with adrain port 44e, and the fluid line 129 is allowed to communicate with afluid line 140 which is connected to the I & II range pressure reductionvalve 72.

The 5-2 relay valve 48 comprises a spring 48a, and a spool 48b biased bythe spring 48a to assume a first or spring set position as illustratedin FIG. 1B. The spool 48b is moveable in response to the hydraulicpressure within the fluid line 126 and urged against the spring 48a forupward movement, as viewed in FIG. 1B, from the illustrated position toa second or upper position. When the spool 48b is in the illustratedposition, the fluid line 125 is allowed to communicate with a drain port48c, while when it is in the upper position thereof, the fluid line 125is isolated from the drain port 48c and allowed to communicate with afluid line 141 which is connected to the 5-2 sequence valve 50.

The 5-2 sequence valve 50 comprises a spring 50a, and a spool 50b biasedby the spring 50a to assume a first or spring set position asillustrated in FIG. 1A. The spool 50b is moveable in response tohydraulic pressure within a fluid line 142. When the hydraulic pressureprevails in the fluid line 142, the spool 50b is urged against thespring 50a for downward movement, as viewed in FIG. 1A, from theillustrated position to a second or lower position. When the spool 50bis in the illustrated position, the fluid line 141 is allowed tocommunicate with a drain port 50c, while when it is in the lowerposition thereof, the fluid line 141 is isolated from the drain port 50cand allowed to communicate with the fluid line 127.

The fluid line 142 is connected to a fluid line 144 which interconnectsthe servo release chamber 3,4S/R of the band brake B/B and the highclutch H/C. An one-way orifice 143 is disposed in that portion of thefluid line 144 which extends between the high clutch H/C and a junctionat which the fluid line 142 is connected to the fluid line 144.Connected to the fluid line 142 is a fluid line 147 having an one-wayorifice and a one-way coupling 146. The fluid line 147 is connected tothe accumulator shift valve 58. Also connected to this accumulator shiftvalve 58 are a fluid line 148 that is connected to the accumulator 56and a fluid line 149 connected to the reverse clutch R/C.

The accumulator shift valve 58 comprises a spring 58a and a spool 58bbiased by the spring 58a to assume a first or spring set position asillustrated in FIG. 1B. The spool 58b is moveable in response tohydraulic pressure within the D-range fluid line 110. When the hydraulicpressure exists in the D-range fluid line 110, the spool 58b is urgedagainst the spring 58a for leftward movement, as viewed in FIG. 1B, to asecond or leftward position. When the spool 58b is in the illustratedposition, the fluid line 148 is allowed to communicate with the fluidline 149 to put the accumulator 56 in use for controlling pressure risein the reverse clutch R/C. When the spool 58b assumes the leftwardposition owing to the presence of the hydraulic pressure within theD-range fluid line 110, the fluid line 148 is isolated from the fluidline 149 and allowed to communicate with the fluid line 149, putting theaccumulator 56 in use for controlling pressure rise in the servo releasechamber 3,4S/R of the band brake B/B.

Connected to the servo apply chamber 5S/A of ther band brake B/B is afluid line 150. The fluid line 150 extends to an overrunning clutchcontrol valve 62. The accumulator 60 and an one-way orifice 151 arearranged in the fluid line 150 between the servo apply chamber 5S/A andthe overrunning clutch control valve 62. The overrunning clutch controlvalve 62 comprises a spring 62a, and a spool 62b biased by the spring62a to assume a first or spring set position as illustrated in FIG. 1B.In response to supply of hydraulic pressure to a chamber 62c, the spool62b is urged against the spring 62a for upward movement, as viewed inFIG. 1B, from the illustrated position to a second or upper position.When the overrunning solenoid 40 is turned ON to close the associateddrain port, the pilot pressure 90 prevailing in the fluid line 90 isdelivered to the chamber 62c. When the overrunning clutch solenoid 40 isturned OFF, the hydraulic fluid is discharged from the chamber 62c. Whenthe spool 62b is in the illustrated position, a fluid line 152 isallowed to communicate with the D-range fluid line 110 and the fluidline 150 is allowed to communicate with a drain port 62d. When the spool62b assumes the upper position thereof, the fluid line 152 is isolatedfrom the D-range fluid line 110 and allowed to communicate with thedrain port 62d, and the fluid line 150 is isolated from the drain port62d and allowed to communicate with the fluid line 126.

The third shift valve 46 comprises a spring 46a, and a spool 46b biasedby the spring 46a to assume a first or spring set position asillustrated in FIG. 1B. The spool 46b has one end exposed to a chamber46c which is supplied with the pilot pressure from the fluid line 90 andan opposite end exposed to a chamber 46d adapted to be subject tohydraulic fluid pressure within a fluid line 153. The fluid line 153 isconnected to an outlet port of the shuttle valve 154 which has two inletports, one being supplied with the pilot pressure prevailing in thefluid line 90 when the third shift solenoid C is turned ON to close theassociated drain port, the other being supplied with a reverse rangepressure prevailing in the reverse-select fluid line 155. When one ofthese two pressures appears, it is delivered via the shuttle valve 154and the fluid line 153 to the chamber 46d, urging the spool 46b upwards(viewing in FIG. 1B) in assisting the action of the spring 46a to keepthe spool 46b in the illustrated position against the pilot pressurewithin the chamber 46c. When no pressure is delivered to the chamber46d, the spool 46b is urged by the pilot pressure delivered to thechamber 46c for downward movement, as viewed in FIG. 1B, against theaction of the spring 46a. When the spool 46b is in the illustratedposition, a fluid line 156 is allowed to communicate with a drain port46e, and a fluid line 157 is allowed to communicate with the linepressure fluid line 81. When the spool 46b moves downwards to assume asecond or lower position, the fluid line 156 is isolated from the drainport 46e and allowed to communicate with the line pressure fluid line81, and the fluid line 157 is isolated from the line pressure fluid line81 and allowed to communicate with a drain port 46f.

The fluid line 156 is connected to the direct clutch D/C. A one-wayorifice 158 is fluidly disposed in the fluid line 156 between the 5-2relay valve 48 and the direct clutch D/C. Via a one-way orifice 159, adirect clutch accumulator 70 is fluidly connected to the fluid line 156at a portion between the one-way orifice 158 and the direct clutch D/C.The fluid line 157 is connected to the reduction brake RD/B and has aone-way orifice 160 disposed between the third shift valve 46 and thereduction brake RD/B. The reduction brake accumulator 68 is connected tothe fluid line 157 at a portion between the one-way orifice 160 and thereduction brake RD/B.

This accumulator 68 has the fluid line 161 communicating with a back-uppressure chamber thereof. The fluid line 161 extends from theaccumulator 68 and is connected to an outlet port of a shuttle valve112. This shuttle valve 112 has two inlet ports, one being connected tothe III-range pressure fluid line 111 to receive the III-range pressure,the other being connected to a fluid line 155 connected to thereverse-select pressure fluid line 88 to receive the reverse-selectpressure.

The fluid line 161 extends also to the reduction timing valve 66 andcommunicating with a chamber 66a to supply the fluid pressure therein tothe chamber 66a. This reduction timing valve 66 comprises a spool 66bbiased by a spring 66c to assume a first or spring set position asillustrated in FIG. 1B. When the hydraulic pressure is present in thechamber 66a, the spool 66b is urged against the spring 66c for upwardmovement, as viewed in FIG. 1B, from the illustrated position to asecond or upper position thereof. When the spool 66b is in theillustrated position, a bypass line 163 arranged in parallel to theone-way orifice 160 is closed. The bypass line 163 has one end connectedto the fluid line 157 at a portion between the reduction brake RD/B andthe accumulator 68 and an opposite end connected to the reduction timingvalve 66. Thus, when the spool 66a assumes the illustrated position, theopposite end of the branch line 163 is closed and disconnected from thefluid line 157 at a portion between the one-way orifice 160 and thethird shift valve 46. An orifice 162 is disposed in the bypass line 163.When the spool 66b takes the upper position thereof, the opposite end ofthe fluid line 163 is connected to the fluid line 157 at the portionbetween the one-way orifice 160 and the third shift valve 46, thusopening the bypass line 163.

The overrunning clutch reduction valve 64 has a function to reduce thehydraulic pressure supplied thereto from the fluid line 152. The reducedpressure generated by the overrunning clutch reduction valve 152 issupplied to the overrunning clutch OR/C. This reduction valve 64comprises a spring 64a an a spool 64b biased by the spring 64a. Thespool 64b has a pressure acting area exposed to the accumulator back-uppressure from the fluid line 116. The accumulator back-up pressureacting on the spool 64b induces a force urging the spool 64b upwards, asviewed in FIG. 1B, assisting the action of the spring 64a. Thus, thespool 64b assumes a first or spring set position as illustrated in FIG.1B when no hydraulic pressure exists in the fluid line 152. When thespool 64b is in the illustrated position, an outlet port 64c is allowedto communicate with the fluid line 152, allowing supply of hydraulicpressure from this outlet port 64c to the overrunning clutch OR/C. Afluid line 165 with an orifice 164 interconnects this outlet port 64cand the overrunning clutch OR/C. The hydraulic pressure supplied to theoverrunning clutch OR/C is fed to a feedback chamber 64e through anorifice passage 64b which the spool 64b is formed with. As the hydraulicpressure supplied to the overrunning clutch OR/C increases, the spool64b is urged for downward movement from the illustrated position sincethe spool 64b has an upper end, as viewed in FIG. 1B, exposed to thefeedback chamber 64e. When this pressure exceeds a predetermined valuethat is determined by the sum of a force with which the accumulatorback-up pressure urges the spool 64b and a force with which the spring64a biases the spool 64b, the spool 64b assumes a position where itallows the outlet port 64c to communicate with a drain port 64f, causinga drop in the hydraulic pressure. As a result, the hydraulic pressuresupplied to the overrunning clutch OR/C becomes equal to thepredetermined value. However, since the accumulator back-up pressureincreases as the engine load increases, this predetermined value alsoincreases as the engine load increases. Thus, the hydraulic pressuresupplied to the overrunning clutch OR/C increases as the engine loadincreases. As a result, a transient capacity of the overrunning clutchOR/C during engagement thereof is variable with variation in the engineload. A fluid line 167 with a one-way valve 166 is arranged in parallelto the orifice 164 and has one end communicating with the fluid line 152and an opposite end connected to the fluid line 165. The one-way valve166 and the orifice 164 cooperate with each other to form an one-wayorifice.

The II-range pressure reduction valve 72 has a function to effect apressure reduction of the hydraulic pressure supplied thereto from theII-range fluid line 113 and delivers the reduced pressure to the fluidline 140. The reduction valve 72 comprises a spring 72a and a spool 72bbiased by the spring 72a to assume a spring set position as illustratedin FIG. 1B. When the spool 72b is in this illustrated position, thefluid line 113 is allowed to communicate with the fluid line 140,inducing a hydraulic pressure within the fluid line 140. This hydraulicpressure acts via an orifice passage 72 on the righthand end of thespool 72b, urging the spool 72b for leftward movement, as viewed in FIG.1B, against the action of the spring 72a. When the hydraulic fluidpressure acting on the righthand end of the spool 72b exceeds apredetermined value that is determined by the force of the spring 72a,the fluid line 140 is allowed to communicate with a drain port 72d,allowing drainage of a portion of hydraulic fluid, causing a drop in thehydraulic pressure. As a result, the hydraulic fluid pressure within thefluid line 140 becomes equal to the predetermined value.

The operation of the hydraulic control system shown in FIGS. 1A and 1Bis described.

Before entering the description, the states which the shift solenoids A,B, and C should take for each of the five forward speeds or gearpositions can be tabulated as follows:

    ______________________________________                                                     SHIFT SOL.                                                       GEAR           A         B         C                                          ______________________________________                                        FIRST          ON        ON        ON                                         SECOND         OFF       ON        ON                                         THIRD          OFF       OFF       ON                                         FOURTH         OFF       OFF       OFF                                        FIFTH          ON        OFF       OFF                                        ______________________________________                                    

P or N

When a driver places the manual valve 38 to P position or N positionwhile the vehicle is at a standtill, all of the ports 38D, 38III, 38II,and 38R are allowed to communicate with a drainage and thus serve asdrain ports, respectively. Thus, the fluid line 81 is not supplied tothe forward clutch F/C, high clutch H/C, band brake B/B, reverse clutchR/C, low & reverse brake LR/B, and overrunning clutch OR/C are helddeactivated. As a result, the main gearing 3 is conditioned in neutral.

Under this condition, the auxiliary gearing 4 is held in the low gearposition thereof for simpliicaion of the valve assembly. The third shiftsolenoid C is turned ON when the manual valve 38 is placed at P or Nposition, the pilot pressure prevails in the fluid line 153 and thusreaches the chamber 46d of the third shift valve 46. Thus, the thirdshift valve 46 assumes the illustrated position wherein the fluid line156 is allowed to communicate with the drain port 46e and the fluid line157 is allowed to communicate with the line pressure fluid line 81. As aresult, the direct clutch D/C is drained via the fluid line 156, and thereduction brake RD/B is engaged with the line pressure supplied theretothrough the fluid line 157, conditioning the auxiliary gearing 4 in thelow gear position.

D RANGE

When the manual valve 38 is placed at D position, an automatic shiftamong the five gear positions is effected.

FIRST GEAR

With the manual valve 38 placed at D position, the port 38D only isallowed to communicate with the line pressure fluid line 81, allowingthe line pressure to prevail in the fluid line 110 and to serve as aso-called D range pressure. Immediately after the manual valve 38 hasbeen shifted from P or R position to D position, the hydraulic fluidflows through the fluid line 110 toward the forward clutch F/C. Sincethis flow of hydraulic fluid is restricted by the one-way orifice 120and influenced by the N-D accumulator 54, the forward clutch F/C isengaged without any substantial shock.

When the manual valve 38 is shifted from P or N position to D positionwhile the vehicle is at a standstill, the third shift solenoid C is keptturned ON, and the first and second shift solenoids A and B are turnedON under the control of the computer. With the first and second shiftsolenoids A and B turned ON, the spools 42b and 44b of the first andsecond shift valves 42 and 44 are shifted to their upper positions,respectively. With the spool 44b of the second shift valve 44 in theupper position thereof, the fluid line 128 extending to the high clutchH/C is allowed to communicate with the drain port 44e. Since the fluidline 128 is drained, the high clutch H/C is held disengaged or released.Since it is connected to the fluid line 128, the fluid line 144 is alsodrained. Thus, the servo release chamber 3,4S/R for the band brake B/Bis drained. The servo apply chamber 2S/A for the band brake B/B isdepressurized since the fluid line 122 is allowed by the first shiftvalve 42 to communicate with the fluid line 128 which is drained underthis condition. The other servo apply chamber 5S/A for the band brakeB/B is depressurized since the fluid line 150 is drained in thefollowing manner. Unless the driver manipulates to command an enginebraking, the computer keeps the overrunning clutch solenoid 40 turnedON, so that the pilot pressure is supplied to the overrunning clutchcontrol valve 62 to hold the spool 62b thereof in the upper position.This causes the fluid line 152 to communicate with the drain port 62d,releasing the overrunning clutch OR/C. This also causes the fluid line150 to communicate with the fluid line 126. Since this fluid line 126 isallowed by the first shift valve 42 to communicate with the fluid line128 that is drained via the drain port 44e owing to the second shiftvalve 44. Thus, the fluid line 150 is drained and the servo applychamber 5S/A is also drained. The reverse clutch R/C and the low &reverse brake LR/B are both released since the fluid line 88 is drainedvia the drain port 38R of the manual valve 38.

Since the reduction gear is established in the auxiliary gearing 3, theforward clutch F/C only is engaged and the forward one-way clutch F/OWCis active, the overall automatic transmission is conditioned in thefirst gear position thereof. Under this condition, the torque converterT/C is held in the converter state. Owing to the positions taken by thefirst and second shift valves 42 and 44, the fluid line 108 communicateswith the fluid line 127 which in turn communicates with the D-rangefluid line 110, so that the line pressure in the D-range fluid line 110is delivered through the fluid lines 127 and 108, shuttle valve 107, andthe fluid line 106 to the lock-up control valve 34. This causes the plug34c to hold the spool 34a in the upper position thereof whereby thetorque converter T/C is conditioned in the converter state. Accordingly,the engine is able to keep running without engine stall with the overallautomatic transmission being conditioned in the first gear position andserves as a brake holding the vehicle at a standstill.

Shock upon engagement of the forward clutch F/C is alleviated owing tothe one-way orifice 120 and N-D accumulator 54. Thus, this arrangementalleviates a so-called N-D select shock taking place upon shifting themanual valve 38 from N or P position to D position.

With the automatic transmission conditioned in the first gear positionor first speed, the vehicle starts moving forward when the driverdepresses the accelerator pedal.

SECOND GEAR

When the vehicle increases its speed and enters an operating conditionwhich demands that the automatic transmission be conditioned in thesecond gear position, the computer turns the first shift solenoid A OFF,causing the first shift valve 42 to take the spring set position asillustrated in FIG. 1A, isolating the fluid line 126 from the fluid line128 and allowing the fluid line 126 to communicate with the fluid line125. This fluid line 125 is allowed to communicate with the drain port48c owing to the 5-2 relay valve 48. Thus, the fluid line 126 is keptdrained, leaving the servo apply chamber 5S/A for the band brake B/Bdrained. When the first shift valve 42 is in the spring set position asillustrated in FIG. 1A, the fluid line 108 is isolated from the fluidline 127 and now allowed to communicate with the drain port 42d,draining the fluid line 106 and in turn the chamber 34g of the lock-upcontrol valve 34. Thus, the plug 34c takes the position as illustratedin FIG. 1A and no longer holds the spool 34 a in the position wherebythe torque converter T/C maintains the converter state. Under thiscondition, the lock-up solenoid 36, under the control of the computer,causes the spool 34a of the lock-up control valve 34 to shift betweenthe position as illustrated in FIG. 1A, whereby the torque converter T/Cis conditioned in the lock-up state and the other position whereby thetorque converter T/C is conditioned in the converter state.

With the first shift valve 42 in the illustrated position, the fluidline 122 is isolated from the fluid line 128 and allowed to communicatewith the D-range pressure fluid line 110, allowing the line pressure tobe supplied to the fluid line 122 as an servo activating pressure forthe servo apply chamber 2S/A. Owing to the before-mentioned function ofthe 1-2 accumulator valve 52, a change, with respect to time, in thehydraulic pressure prevailing in that portion of the fluid line 122which extends between the first shift valve 42 and the one-way valve 130is modulated and then transmitted to the servo apply chamber 2S/A. Thus,the band brake B/B is engaged or applied without any substantial shock,namely, a 1-2 shift shock. With the forward clutch F/C, forward one-wayclutch F/OWC, and reduction brake RD/B held activated, engagement of theband brake B/B causes the automatic transmission to upshift from thefirst gear position to the second gear position.

THIRD GEAR

When the vehicle enters an operating condition which demands that thethird gear be established in the automatic transmission, the computerturns the second shift solenoid B OFF, allowing the second shift valve44 to shift to the position as illustrated in FIG. 1B. With the secondshift valve 44 in the illustrated position, the fluid line 128 isisolated from the drain port 44e and allowed to communicate with theD-range pressure fluid line 110. Thus, the line pressure in the fluidline 110 is transmitted via the one-way orifice to the high clutch H/C,causing the high clutch H/C to be engaged. The hydraulic pressure actingon the high clutch H/C is transmitted to the servo release chamber3,4S/R through the fluid line 144 communicating with that portion of thefluid line 128 which extends between the one-way orifice 132 and thehigh clutch H/C. Thus, the band brake B/B is released owing topressurization of the servo release chamber 3,4S/R. The accumulatorshift valve 58 has its spool 58b moved leftwards against the spring 58aowing to the D-range pressure applied to the righthand end of the spool58b viewing in FIG. 1B. This causes the fluid line 147 to communicatewith the fluid line 148 which in turn communicates with the accumulator56. Thus, the hydraulic fluid to be supplied to the servo releasechamber 3,4S/R is also supplied, via the one-way orifice 145, one-waycoupling 147, fluid line 147 and fluid line 148, to the accumulator 56.This causes the piston of the accumulator 56 to stroke from a positionas illustrated by the righthand half thereof viewing in FIG. 1B to aposition as illustrated by the lefthand half thereof viewing in FIG. 1B.During this stroke of the accumulator piston, the hydraulic pressuresupplied to the servo release chamber 3,4S/R gradually increases. As aresult, the band brake B/B is released in good timed relationship withengagement of the high clutch H/C.

With the forward clutch F/C, forward one-way clutch F/OWC and reductionbrake RD/B held activated, releasing the band brake B/B in good timedrelationship with engagement of the high clutch H/C causes the automatictransmission to upshift from the second gear position to the third gearposition. Owing to the accumulator 56, a shift shock taking place duringthis upshift is suppressed.

FOURTH GEAR

When the vehicle enters an operating condition which demands the fourthgear be established in the automatic transmission, the computer turnsthe third shift solenoid C OFF with the first and second shift solenoidsA and B held OFF. Deenergization of the third shift solenoid C causesthe spool 46b of the third shift valve 46 to move downwards from theillustrated position as viewed in FIG. 1B toward the lower position.When the spool 46b of the third shift valve 46 takes the lower position,the fluid line 157 is isolated from the line pressure fluid line 81 andallowed to communicate with the drain port 46f, while the fluid line 156is isolated from the drain port 46e and allowed to communicate with theline pressure fluid line 81. This causes the reduction brake RD/B to bereleased by draining the fluid line 157 and the direct clutch D/C to beengaged by pressurizing the fluid line 156. As a result, the auxiliarygearing 4 shifts from the low gear position thereof to the high ordirect drive position thereof, causing the automatic transmission toupshift from the third gear position to the fourth gear position.

During this upshift operation, the servo activating hydraulic pressuresupplied to the direct clutch D/C gradually increases owing to theone-way orifice 158 and to stroke of the piston of the direct clutchaccumulator 70 from a position as illustrated by the right half thereofto a position as illustrated by the left half thereof as viewed in FIG.1B. In this manner, a so-called 3-4 shift shock induced by engagement ofthe direct clutch D/C is alleviated.

FIFTH GEAR

When the vehicle enters an operating condition which demands the fifthgear be established in the automatic transmission, the computer turnsthe first shift solenoid A ON, causing the spool 42b of the first shiftvalve 42 to move upwards as viewed in FIG. 1A. When the spool 42b of thefirst shift valve 42 assumes the upper position thereof, the fluid line128 which is allowed to communicate with the D-range pressure line 110by the second shift valve 44 is allowed to communicate with the fluidline 126, allowing the D-range pressure to be supplied to the fluid line126. This hydraulic pressure is supplied via the overrunning clutchcontrol valve 62 and fluid line 150 to the servo apply chamber 5S/A. Asa result, the band brake B/B is applied. Owing to he above-mentionedshift of the first shift valve 42, the fluid line 122 is isolated fromthe D-range pressure fluid line 111 and allowed to communicate with thefluid line 128. Since the D-range fluid pressure is kept supplied viathe fluid line 128 to the fluid line 122, the servo apply chamber 2S/Ais kept pressurized.

With the forward clutch F/C, high clutch H/C and direct clutch D/C beingkept engaged, engagement of the band brake B/B in the previouslydescribed manner causes the automatic transmission to upshift from thefourth gear position to the fifth gear position (overdrive).

During this upshift operation, the flow of hydraulic fluid toward theservo apply chamber 5S/A is restricted by the one-way orifice 151, andthe piston of the accumulator 60 strokes from a position as illustratedby the righthand half thereof to a position as illustrated by thelefthand half thereof as viewed in FIG. 1B. As a result, the band brakeB/B is engaged without any substantial shock and a so-called 4-5 shiftshock is alleviated.

When the spool 42b of the first shift valve 42 shifts from theillustrated position to the upper position thereof, the fluid line 108is isolated from the drain port 42d and allowed to communicate with thefluid line 127. Since this fluid line 127 still communicates with thedrain port 44d when the second shift valve 44 is in the illustratedposition, no hydraulic fluid pressure is supplied to the chamber 34g ofthe lock-up control valve 34, leaving the lock-up control valve 34 underthe control of the solenoid 36.

OD INHIBITING

When the driver closes the overdrive inhibiting switch by pressing abutton thereof, the computer will not select such an ON-OFF combinationof the shift solenoids A and B which causes the automatic transmissionto assume the fifth gear position. Thus, the automatic transmission isshiftable between the first, second, third and fourth gear positions.Under this condition, the computer turns off the overrunning clutchsolenoid 40 when the throttle opening degree becomes smaller than apredetermined value, i.e., 1/16 of the fully opened throttle openingdegree. This causes the overrunning clutch control valve 62 to shift tothe position as illustrated in FIG. 1B. When the overrunning clutchcontrol valve 62 takes the illustrated position as viewed in FIG. 1B,the fluid line 152 is allowed to communicate with the D-range fluid line110. Thus, the D-range pressure is supplied to the fluid line 152 andthen to the overrunning clutch reduction valve 64 where it is subject topressure regulation to generate a reduced pressure. This reducedpressure is supplied to the overrunning clutch OR/C to engage the same.This engagement of the overrunning clutch OR/C causes the automatictransmission to establish engine brake running status with the secondgear position, third gear position and fourth gear position. Since Theoverrunning clutch OR/C is engaged with the reduced hydraulic pressureowing to the reduction valve 64, a shock taking place upon engagement ofthe overrunning clutch OR/C is alleviated.

When the overrunning clutch control valve 62 shifts in theabove-mentioned manner, the fluid line 150 is allowed to communicatewith the drain port 62d, draining the servo apply chamber 5S/A, thusreleasing the band brake B/B. Thus, the band brake B/B is released whenthe overrunning clutch OR/C is engaged, preventing the transmission frominterlocking owing to engagement of the overrunning clutch OR/C uponengagement of the band brake B/B.

4-3 DOWNSHIFT

Description is made as to a shift in the auxiliary gearing 4 from thehigh gear position (direct drive) to the low gear position in effectinga 4-3 downshift.

In effecting the 4-3 downshift, the computer holds the main gearing 3 inthe fourth gear position thereof and shifts the third shift solenoid Cfrom the OFF state to the ON stage, rendering the third shift valve 46to assume the position as illustrated in FIG. 1B. As a result, the fluidline 156 is allowed to communicate with the drain port 46e and thusdrained, allowing the release of the direct clutch D/C. At the sametime, the fluid line 157 is allowed to communicate with the linepressure fluid line 81, allowing the lined pressure to be supplied tothe reduction brake RD/B in the following manner.

During this 4-3 downshift in the D-range, no pressure is supplied to thechamber 66a of the reduction timing valve 66 and the back-up pressurechamber of the reduction brake accumulator 68 since fluid line 161 isdrained. This is because the manual valve 38 has the ports 38III and 38Rdrained, thus draining the fluid lines 111 and 88. As a result, thereduction timing valve 66 assumes the illustrated position where thebypass line 163 is closed and the hydraulic fluid is applied to thereduction brake RD/B only through the one-way orifice 160. Besides, theback-up pressure of the reduction accumulator 68 is zero, so that theaccumulator 68 exhibits a pressure modulating characteristic determinedby the spring 68a.

Referring to FIG. 3, the fully drawn line P_(D) shows how the hydraulicpressure supplied to the reduction brake RD/B varies during the 4-3downshift with the manual valve 38 in D position. Initially, the piston68b of the accumulator 68 assumes the position as illustrated by theright half thereof viewing in FIG. 1B and thus the hydraulic pressuresupplied to the reduction brake RD/B jumps to a value P₁ determined bythe force of spring 68a. Subsequentially, the piston 68b of theaccumulator 68 moves from the position as illustrated by the right halfthereof viewing in FIG. 1B to the position as illustrated by the lefthalf thereof viewing in FIG. 1B against the force of the spring 68a.During this phase, the hydraulic pressure supplied to the reductionbrake RD/B increases at a rate, with respect to time, determined by theinternal diameter of the one-way orifice 160. At a moment t₁ prior tothe termination of this movement of the piston 68b of the accumulator68, the hydraulic pressure supplied to the reduction brake RD/B jumps toa value determined by the force of the return spring (not shown) of thereduction brake RD/B. Then, the reduction brake RD/B is subject to aso-called "lost stroke". At a moment t₂ when this "lost stroke"terminates, the hydraulic pressure supplied to the reduction brake RD/Bjumps to a level as high as the line pressure P_(L).

A pressure value required for engagement of the reduction brake RD/B isP₂ and the reduction brake RD/B is engaged at a moment t₂ when thehydraulic pressure increases up to this value P₂. Since engine brakingis not required during D-range and engine torque is transmitted outputshaft, the reduction one-way clutch RD/OWC (see FIG. 2) serves as areaction element during the release of the direct clutch D/C, Thus, thedelayed engagement of the reduction brake RD/B does not cause anyproblem in shifting operation. Owing to the delayed engagement of thereduction brake RD/B, simultaneous engagement of the reduction brakeRD/B with engagement of the direct clutch D/C can be avoided.

III RANGE

If one wishes engine braking at high vehicle speed, the drivers shiftsthe manual valve 38 to III range position. Then, the computer controlsthe shift solenoids A, B and C in order to shift the automatictransmission among the first, second and third gear positions inaccordance with varying operating condition. The overrunning clutchsolenoid 40 is turned OFF when the engine throttle opening degree issmaller than a predetermined value, for example, 1/16 of the fullyopened throttle opening degree, the overrunning clutch OR/C is engaged.This causes engine braking at the second or third gear position.

If, during running at the fifth or fourth gear position with the manualvalve 38 placed at D position, the driver shifts the manual valve 38 toIII range position, the auxiliary gearing 4 shifts from the high earposition to the low gear position in the same manner as it does during4-3 downshift. However, the manual valve 38 allows the port 38III tooutput the line pressure. Thus, this line pressure is supplied via thefluid line 111, shuttle valve 112, fluid line 161 to the chamber 66a ofthe reduction timing valve 66 and the back-up chamber of the accumulator68. As a result, the spool 66b of the reduction timing valve 66 is urgedby the line pressure supplied to the chamber 66a to assume the upperposition thereof where the bypass line 163 is allowed to communicatewith the fluid line 157 upstream of the one-way orifice 160 and thusopened. Since the bypass line 163 is opened, the hydraulic fluid issupplied also to the orifice 162 of the bypass line 163 and then to thereduction brake RD/B. Since the line pressure is supplied via the fluidline 161 to the back-up chamber of the accumulator 68, the pressuremodulating characteristic revealed by the accumulator 68 is affected notonly by the force of the spring 68a but also by the hydraulic pressuresupplied to the back-up chamber of the accumulator 68.

Referring to FIG. 3, two-dot chain P_(E) shows the variation inhydraulic pressure supplied to the reduction brake RD/B during shiftingof the auxiliary gearing 4 from the high gear position to the low gearposition after the manual valve 38 has been shifted to III rangeposition. Initially, the hydraulic pressure supplied to the reductionbrake RD/B jumps to a value determined by the return spring of thereduction brake RD/B, causing the servo piston of the reduction brakeRD/B to stroke. Subsequently, the hydraulic pressure supplied to thereduction brake RD/B jumps to a value that is determined by the sum ofthe force of the spring 68a and the force due to the line pressuresupplied to the back-up chambers of the accumulator 68. Then thehydraulic pressure increases at a rate, with respect to time, that isdetermined by the sum of the internal diameter of the one-way orifice160 and the internal diameter of the orifice 162. Upon termination ofthe stroke of the accumulator piston 68a, the hydraulic pressuresupplied to the reduction brake RD/B jumps to a value as high as theline pressure P_(L). Since, the hydraulic pressure supplied to thereduction brake RD/B reaches the value P₂ at a moment t₀ prior to themoment t₂ owing to the additional flow supply through the bypass line163 and the presence of the back-up pressure in the back-up chamber ofthe accumulator 68, the reduction brake RD/B is engaged quickly.

Since engine braking is urgently demanded when the driver shifts themanual valve 38 to III range position, this demand is not met if theengagement of the reduction brake RD/B is delayed from the release ofthe direct clutch D/C. However, as discussed above, the reduction brakeRD/B is quickly engaged in timed relationship with the release of thedirect clutch D/C. As shown in FIG. 3, the moment t₀ when the engagementof the reduction brake RD/B is initiated occurs while the accumulatorpiston 68b is in the process of its stroke, the rate at which thehydraulic pressure supplied to the reduction brake RD/B is not highenough to cause an engagement shock of the reduction brake RD/B. Thus,the shift is effected without any substantial shock.

II RANGE

If one wishes engine braking at the second or first gear position, thedriver shifts the manual valve 38 to II range position. Then, thecomputer controls the shift solenoids A, B and C to cause the automatictransmission to shift between the second and first gear positions. Theoverrunning clutch solenoid 40 is turned OFF when the throttle openingdegree becomes smaller than the predetermined value, namely 1/16 of thefully opened throttle opening degree, causing the overrunning clutchOR/C to be engaged. The manual valve 38 allows port 38II to output linepressure with the port 38III keeping one outputting the line pressure.The line pressure is supplied from the port 38II to the pressurereduction valve 72 where a pressure reduction is effected. The reducedpressure from this valve 72 is supplied to the fluid line 140 as a lowreverse brake pressure.

During running at the first gear position, the first and second shiftvalves 42 and 44 have their spools 42b and 44b held in their upperpositions, respectively, the hydraulic pressure within the fluid line140 is supplied via the fluid lines 129, 124 and shuttle valve 115 tothe low reverse brake LR/B, causing same to be engaged. As a result, theautomatic transmission establishes engine brake running status at thefirst gear position. Since the hydraulic pressure supplied to the Lowreverse brake LR/B is reduced upon establishing the first gear, anengagement shock of the low reverse brake LR/B is suppressed althoughthe low reverse brake LR/B has a large capacity required for reversedrive.

During running at the second gear position, the first shift valve 42takes the illustrated position viewing in FIG. 1A, the fluid line 124 isisolated from the fluid line 129 and allowed to communicate with thedrain port 42f, releasing the low reverse brake LR/B. Since, under thiscondition, the overrunning clutch OR/C is kept engaged, the automatictransmission establishes engine brake running status at the second gearposition.

During running at the fifth gear position, if the driver shifts themanual valve 38 to II range position, the automatic transmission shiftsfrom the fifth gear position directly to the second gear positionbypassing the fourth and third gears. This 5-2 downshift requires thatthe auxiliary gearing 4 shift from the high gear position to the lowgear position in addition to a shift in gear position in the maingearing 3. Detailed description regarding the shift effected in theauxiliary gearing 4 is hereby omitted since the shifting process is thesame as that upon manipulating the manual valve 38 from D range positionto II range position. Thus, the following description concentrates onthe shifting process taking place in the main gearing 3.

Assuming now that the automatic transmission is at the fifth gearposition, the first shift solenoid is turned ON to cause the spool 42bof the first shift valve 42 to take the upper position thereof, and thesecond shift solenoid B is turned OFF to cause the spool 44b of thesecond shift valve 44 to take the illustrated position viewing in FIG.1B, and the overrunning clutch solenoid 40 is turned ON, causing thespool 62b of the overrunning clutch control valve 62 to take the upperposition thereof viewing in FIG. 1B. As a result, the forward clutchF/C, high clutch H/C, servo apply chamber 2S/A, servo release chamber3,4S/R and servo apply chamber 5S/A are supplied with hydraulicpressures. This causes the automatic transmission to establish the fifthgear. Under this condition, the hydraulic pressure supplied to the servorelease chamber 3,4S/R is also supplied via the fluid line 144 and thefluid line 142 to the 5-2 sequence valve 50, holding the spool 50b ofthis valve 50 to the lower position thereof viewing in FIG. 1A. Thehydraulic pressure supplied to the servo apply chamber 5S/A is suppliedvia the fluid line 150 to the overrunning clutch control valve 62. Sincethe fluid line 126 is allowed to communicate with the fluid line 150,this hydraulic pressure is supplied via the fluid line 126 to the 5-2relay valve 48, holding the spool 48b of this valve 48 to the upperposition thereof viewing in FIG. 1B.

Under this condition, if the driver shifts the manual valve 38 from Drange position to II range position, the computer turns the first shiftsolenoid A OFF, causing the spool 42b of the first shift valve 42 toshift to the position as illustrated in FIG. 1A, and the second shiftsolenoid B ON causing the spool 44b of the second shift valve 44 toshift to the upper position thereof viewing in FIG. 1B. However, theoverrunning clutch solenoid 40 is kept ON until the 5-2 downshift iscompleted. Thus, the spool 62b of the overrunning clutch control valve62 is held in the upper position thereof. The above-mentioned shift ofthe second shift valve 44 causes discharge of the hydraulic fluid fromthe servo release chamber 3,4S/R and high clutch H/C. However, owing tothe provision of the one-way orifice 131 in the fluid line 128 and theone-way orifice 143 in the fluid line 144, the hydraulic fluid isdischarged at a gradual rate. Thus, the hydraulic pressure remaining inthe servo release chamber 3,4S/R holds the spool 50b of the 5 -2sequence valve 50 to the lower position thereof, keeping the fluidcommunication between the fluid lines 127 and 141. Owing to theabove-mentioned shift of the second shift valve 44, the fluid line 127is allowed to communicate with the D range pressure fluid line 110,allowing the D range pressure to be supplied via the fluid line 167, 5-2sequence valve 50, fluid line 141, 5-2 relay valve 48, fluid line 125,first shift valve 42, fluid line 126, overrunning clutch control valve62, and fluid line 150 to the servo apply chamber 5S/A. In this manner,the hydraulic pressure supplied to the servo apply chamber 5S/A isbacked up regardless of the states of the shift solenoids A and B. Thispressure back-up is held since the hydraulic pressure supplied to theservo apply chamber 5S/A acts on the bottom end of the spool 48b of the5-2 relay valve 48 to hold the spool 48b of this valve 48 to the upperposition thereof.

Subsequently, when the servo apply chamber 3,4S/R is drained, the spool50b of the 5-2 sequence valve 50 moves back to the illustrated positionviewing in FIG. 1A owing to the action of the spring 50a, allowing thefluid line 141 to communicate with the drain port 50c. As a result, thehydraulic fluid supplied to the servo apply chamber 5S/A to back uppressure therein is now discharged via the fluid line 150, fluid line126, fluid line 125, fluid line 141, and drain port 50c of the 5-2sequence valve 50. This causes the spool 48b of the 5-2 relay valve 48to assume the illustrated position viewing in FIG. 1B. In the previouslydescribed manner, the servo apply chamber 5S/A is drained after theservo release chamber 3,4S/R has been drained. This keeps the band brakeB/B engaged during the 5-2 downshift. Therefore, the automatictransmission shifts from the fifth gear position to the second gearposition directly without establishing the fourth nor the third gear.

Upon completion of the 5-2 downshift operation, the computer turns theoverrunning clutch solenoid 40 OFF, causing the overrunning clutchcontrol valve 62 to shift to the illustrated position viewing in FIG.1B. This causes the fluid line 150 for the servo apply chamber 5S/A tocommunicate with the drain port 62d and the fluid line 152 for theoverrunning clutch OR/C to communicate with the D range pressure fluidline 110. Thus, the D range pressure is supplied to the overrunningclutch OR/C to engage same. This engagement of the overrunning clutchOR/C ensures engine braking. From the preceding description, it will nowbe appreciated that the 5-2 downshift bypassing the fourth and thirdgears is assured when the driver shafts the manual valve 38 to II rangeposition, thus producing great engine braking demanded by the driver.

The 5-2 sequence valve 50 is rendered operable to perform theabove-mentioned function only when the spool 48b of the 5-2 relay valve48 is held in the upper position thereof owing to the presence ofhydraulic pressure within the servo apply chamber 5S/A during forwarddrive at the fifth gear position. Thus, since the 5-2 sequence valve 50is rendered inoperable, the back-up function of the hydraulic pressurewithin the servo apply chamber 5S/A performed by the 5-2 sequence valve50 is avoided owing to the provision of the 5-2 relay valve 48.

I RANGE

If one wishes engine braking at the first gear position, the drivershifts the manual valve 38 to II range position and then turns on a Irange switch, not illustrated. Then, the computer turns the shiftsolenoids A, B, and C ON, and turns the overrunning clutch solenoid 40OFF when the engine throttle opening degree becomes smaller than thepredetermined value, namely 1/16 of the fully opened throttle openingdegree. This causes the automatic transmission to shift to the firstgear position and establishes engine braking status when the throttleopening degree becomes smaller than the predetermined throttle openingdegree.

R RANGE

If one wishes that the vehicle travel in the reverse direction, thedriver shifts the manual valve 38 to R (reverse) position wherein theport 38R only is allowed to communicate with the fluid line 81 to allowoutput of the line pressure therefrom, and all of the other ports aredrained. The line pressure appearing at the port 38R is supplied to thefluid line 88 as the reverse-select hydraulic pressure. The hydraulicpressure within the fluid line 88 is supplied via the shuttle valve 107,fluid line 106 to the chamber 34g of the lock-up control valve 34. Thiscauses the valve 34 to assume the position wherein the torque converterT/C is conditioned to be operable in the torque converter state.

On the other hand, the hydraulic pressure within the fluid line 88 issupplied to the fluid line 155. Then, this hydraulic pressure within thefluid line 155 is supplied via the shuttle valve 154 and fluid line 153to the chamber 46d of the third shift valve 46, urging this valve 46 tothe position as illustrated in FIG. 1B. In this illustrated position ofthe third shift valve 46, the direct clutch D/C is released and thereduction brake RD/B is engaged to condition the auxiliary gearing 4 inthe low gear position thereof. The hydraulic pressure within the fluidline 155 is supplied via the shuttle valve 112 and fluid line 161 to thechamber 66a of the reduction timing valve 66 and also to the back-upchamber of the reduction brake accumulator 68. This causes the reductiontiming valve 66 and the reduction brake accumulator 68 to perform thesame functions as they did during III range. Thus, the reduction brakeRD/B is quickly engaged to condition the auxiliary gearing 4 in the lowgear position thereof without any substantial shock although thereduction one-way clutch RD/OWC does not assist in holding the third sungear 7_(S) (see FIG. 2). The reduction one-way clutch RD/OWC does notserve as a reaction element during transient phase since the third sungear 7_(S) tends to rotate in the same direction as it is during enginebraking.

When the manual valve 38 is placed at N or P position, the line pressureis supplied from the line pressure fluid line 81 to the fluid line 157leading to the reduction brake RD/B. Thus, it may be considered that thefunctions to be performed by the reduction timing valve 66 and reductionaccumulator 68 for the purpose of quick engagement of the reductionbrake RD/B does not mean anything upon shifting the auxiliary gearing 4after the manual valve 38 has been shifted from N or P position to Rposition. However, the reduction timing valve 66 and reductionaccumulator 68 proves its effectiveness when the manual valve 38 isplaced at R position immediately after the engine has been started withthe manual valve 38 placed at N or P position. Under this condition, theauxiliary gearing 4 remains in the neutral state owing to a delay isrising of hydraulic pressure, and thus it is shifted to the low gearposition after the reduction brake RD/B is engaged by the hydraulicpressure supplied thereto via the fluid line 157. If there is aconsiderable period of time until the RD/B is engaged, the auxiliarygearing 4 stays in the neutral state for the considerable period oftime, causing a considerable delay until the overall automatictransmission is conditioned for the reverse drive when the manual valve38 is shifted to R position immediately after the engine has beenstarted with the manual valve 38 placed at N or P position. This causesnot only a delay until the vehicle starts moving, but also an engineracing. The engine racing induces a large shock upon selecting Rposition. These problems become serious when the manual valve 38 isshifted to R position immediately after the engine has been startedunder cold weather with the manual valve 38 placed at N or P position,since the transmission oil increases its viscosity when the environmenttemperature is very low.

Owing to the arrangement of the reduction timing valve 66 andaccumulator 68, the reduction brake RD/B is engaged quickly whenever themanual valve 38 is shifted to R position, thus solving the above-listedproblems.

The hydraulic pressure within the fluid line 88 is supplied via theone-way orifice 114 and shuttle valve 115 to the low reverse brake LR/Bto engage same, and it is also supplied via the one-way orifice 117 tothe reverse clutch R/C to engage same. The other frictional devices ofthe main gearing 3, namely, the forward clutch F/C, high clutch H/C,band brake B/B, and overrunning clutch OR/C are all drained since theyreceive hydraulic fluid from the ports 38D, 38III, and 38II of themanual valve 38 which are now drained. Thus, the automatic transmissionis conditioned in the reverse drive owing to engagement of the lowreverse brake LR/B, reverse clutch R/C, and reduction brake RD/B. Theshift solenoids A, B, and C, and overrunning clutch solenoid 40 may beturned ON or OFF when the automatic transmission is in the reverse gearposition; However, if they are left turned OFF, the hydraulic fluid iskept drained via the associated drained ports of these solenoids,causing an energy loss in driving the oil pump O/P. Thus, the computeris so programmed as to turn the shift solenoid A, B, C and theoverrunning clutch solenoid 40 ON when the manual valve 38 is placed atR position.

The accumulator shift valve 58 is not receiving the hydraulic pressurefrom the D-range pressure fluid line 110 when the manual valve 38 isplaced at R position. This causes the fluid line 149 to communicate withthe fluid line 148, rendering the accumulator 56 operable in controllingincrease in hydraulic pressure supplied to the reverse clutch R/C whenit is to be engaged. The hydraulic fluid directed toward the reverseclutch R/C is restricted by the one-way orifice 117 and then admittedvia the fluid line 149, accumulator shift valve 58 and fluid line 148 tothe accumulator 56. The hydraulic pressure thus increases graduallywhile the piston of the accumulator 56 is in the stroke against theforce due to the hydraulic pressure within the line pressure fluid line81. Thus, an engagement shock of the reverse clutch R/C is alleviated,resulting in reduction in a shock taking place when the manual valve 38is shifted from P or N position to R position.

From the preceding description, it is appreciated that all of the threeshift solenoids A, B, and C are turned ON or energized to close theirassociated drain ports for establishing the first gear position.According to this control strategy, drainage of hydraulic fluid from thedrain ports associated with these shift solenoids A, B, and C isprevented when the automatic transmission is set in the first gearposition. Since the engine idle, and the flow rate of hydraulic fluiddischarged by the pump O/P is at a low level, it is preferrable toprevent drainage of hydraulic fluid via the drain ports associated withthe shift solenoid A, B, and C when the automatic transmission is set inthe first gear position.

Each of the shift solenoids A, B, and C is a component of anelectro-hydraulic transducer using the pilot pressure in the fluid line90 as an input and generating, as an output, a solenoid hydraulicpressure. The electro-hydraulic transducer is of the off drain type thatgenerates the solenoid hydraulic pressure when its solenoid A or B or Cis energized.

Referring to FIGS. 1A and 1B, attention should now be paid to thefollowing components, namely, the regulator valve 20, shift valve 46connected to the regulator valve 20 via the line pressure fluid line 81,pilot valve 22 generating the pilot pressure which is supplied via thefluid line 90 to act on the upper end of the spool 46b of the shiftvalve 46, electro-hydraulic transducer of the off drain type includingthe shift solenoid C, manual valve 38 which generates the reverse-selecthydraulic pressure in the fluid line 155, and shuttle valve 154 whichallows supply of the solenoid hydraulic pressure to the shift valve 46through the fluid line 153 or supply of the reverse-select hydraulicpressure to the shift valve 46 through the fluid line 153. The solenoidhydraulic pressure or reverse-select hydraulic pressure acts on thelower end of the spool 46b of the shift valve 46. FIG. 1B shows theshift valve 46 in a low gear position thereof. The spring 46a acts onthe spool 46b to bias same toward the low gear position thereof. Thesolenoid hydraulic pressure urges the spool 46b toward the low gearposition thereof. The pilot pressure acting on the upper end of thespool 46b urges same against the spring 46a downwards, as viewed in FIG.1B, toward high gear position thereof. The reverse-select hydraulicpressure acting on the lower end of the spool 46b urges the spool 46btoward the low gear position thereof.

Since the reverse-select hydraulic pressure is higher than the pilotpressure, the shift valve 46 is held at the low gear position thereofwhen the automatic transmission has the reverse drive range. Thus, evenif the shift solenoid C is inadvertently deenergized, the auxiliarygearing 4 is held at the low gear position thereof.

If the shift solenoid C is inadvertently deenergized and the solenoidhydraulic pressure disappears when the automatic transmission has theforward drive range, the pilot pressure urges the spool 46b to moveagainst the spring 46a to the high gear position since the spring 46a isweaker than the opposing force owing to the pilot pressure. Thus, theauxiliary gearing 4 is held at the high gear position, preventing engineoverrunning.

Referring to FIG. 4, the second embodiment is described. FIG. 4 shows ashift valve 46A in a high gear position thereof wherein aservo-activating line pressure P_(L) generated by a regulator valve 20is supplied to a direct clutch D/C and a reduction brake RD/B isdrained, A spring 46g arranged in drain chamber 46c urges a spool 146btoward the high gear position. A reverse-select hydraulic pressure P_(R)generated by a manual valve 38 is supplied via a shuttle valve 154 to aport 46d, urging the spool 146b to move against the spring 46g upwards,viewing in FIG. 4, to a low gear position thereof. A solenoid hydraulicpressure P_(S) generated by an electro-hydraulic transducer of the offdrain type is supplied via the shuttle valve 154 to the port 46d to urgethe spool 146b toward the low gear position thereof.

Since the spring 46g is weaker than the opposing force owing to thereverse-select hydraulic pressure P_(R), the shift valve 46 is held atthe low gear position thereof when the automatic transmission has thereverse drive range.

If a solenoid of the electro-hydraulic transducer is inadvertentlydeenergized and the solenoid hydraulic pressure P_(S) disappears whenthe automatic transmission has the forward drive range, the spring 46gsets the spool 146b to the illustrated high gear position.

Referring to FIG. 5, the third embodiment is described. In thisembodiment, an electro-hydraulic transducer of the on drain type isemployed. This transducer includes a solenoid and generates a solenoidhydraulic pressure P_(S) when the solenoid is turned OFF or deenergized.FIG. 5 shows a shift valve 46B in its low gear position. A spring 46abiases a spool 46b toward the low gear position thereof. Areverse-select hydraulic pressure P_(R) henerated by a manual valve 38is supplied to a port 46d to urge the spool 46b toward the low gearposition thereof. The solenoid hydraulic pressure P_(S) generated by theelectro-hydraulic transducer of the on drain type is supplied to a port46c to urge the spool 46b against the spring 46a toward a high gearposition thereof.

When the automatic transmission has the reverse drive range, thereverse-select hydraulic pressure P_(R) holds the shift valve 46B in thelow gear position as illustrated.

If the solenoid of the electro-hydraulic transducer of the on drain typeis inadvertently deenergized when the automatic transmission has theforward drive range, the solenoid hydraulic pressure P_(S) appears toact on the spool 46b, urging the spool 46b to move against the spring46a to the high gear position.

FIG. 6 shows the fourth embodiment. This embodiment is different fromthe third embodiment illustrated in FIG. 5 only in that a solenoidhydraulic pressure P_(S) generated by an electro-hydraulic transducer ofthe off drain type is supplied to a port 46c of a shift valve 46C tourge a spool 46b for movement against a spring 46a toward a high gearposition thereof.

When the automatic transmission has the reverse drive range, areverse-select hydraulic pressure P_(R) holds the spool 46b of the shiftvalve 46C to a low gear position as illustrated in FIG. 6. Thus, theauxiliary gearing 4 is held at the low gear position thereof when theautomatic transmission has the reverse drive range.

What is claimed is:
 1. An automatic transmission for a motor vehiclewith an engine, the automatic transmission having a forward drive rangeand a reverse drive range, the automatic transmission comprising:a maingearing shiftable between a plurality of gear positions when theautomatic transmission is in the forward drive range and shiftable to areverse gear position when the automatic transmission is in the reversedrive range; an auxiliary gearing drivingly connected to said maingearing, said auxiliary gearing having a high gear position wherein afirst frictional device is engaged and a second frictional device isdisengaged and a low gear position wherein said first frictional deviceis disengaged and said second frictional device is engaged, saidauxiliary gearing bearing shiftable between said high gear position andsaid low gear position; means for generating a servo activatinghydraulic pressure; electro-hydraulic transducer means for generating asolenoid hydraulic pressure; a shift valve fluidly disposed between saidhydraulic pressure generating means, said first frictional device, andsaid second frictional device, said shift valve being shiftable inresponse to said solenoid hydraulic pressure signal to a high gearposition wherein said servo activating hydraulic pressure is supplied tosaid first frictional device and second frictional device is drained,and to a low gear position wherein said first frictional device isdrained and said servo activating hydraulic pressure is supplied to saidsecond frictional device; means for acting on said shift valve in such amanner that, when said electro-hydraulic transducer means normallyoperate, said shift valve shifts to said high and low gear positionsselectively in accordance with a predetermined schedule when theautomatic transmission is in the forward drive range; and means forholding said shift valve at said low gear position thereof when theautomatic transmission is in the reverse drive range, whereby said shiftvalve is held in said low gear position insuring disengagement of saidfirst frictional device if said electro-hydraulic transducer means failsto operate normally.
 2. An automatic transmission as claimed in claim 1,wherein if said electro-hydraulic transducer means fails to operatenormally when the automatic transmission has the forward drive position,said acting means functions to allow said shift valve to be held at saidhigh gear position.
 3. An automatic transmission as claimed in claim 1,wherein said electro-hydraulic transducer means includes a solenoid andis of the off drain type that fails to generate said solenoid hydraulicpressure when said solenoid is deenergized.
 4. An automatic transmissionas claimed in claim 3, wherein said solenoid hydraulic pressure urgessaid shift valve toward said low gear position thereof, and said actingmeans includes spring means for biasing said shift valve toward said lowgear position thereof, and means for generating a pilot hydraulicpressure acting on said shift valve to urge said shift valve toward saidhigh gear position thereof against said spring means, whereby if saidsolenoid of said electro-hydraulic transducer means is inadvertentlydeenergized when the automatic transmission has the forward drive range,said acting means functions to hold said shift valve at said high gearposition thereof.
 5. An automatic transmission as claimed in claim 3,wherein said solenoid hydraulic pressure urges said shift valve towardsaid low gear position thereof, and said acting means includes springmeans for biasing said shift valve toward said high gear positionthereof, whereby if said solenoid of said electro-hydraulic transducermeans is inadvertently deenergized when the automatic transmission hasthe forward drive range, said acting means functions to hold said shiftvalve at said high gear position thereof.
 6. An automatic transmissionas claimed in claim 1, wherein said electro-hydraulic transducer meansincludes a solenoid and is of the on drain type that generates saidsolenoid hydraulic pressure when said solenoid is deenergized.
 7. Anautomatic transmission as claimed in claim 6, wherein said solenoidhydraulic pressure urges said shift valve toward said high gear positionthereof, and said acting means includes spring means for biasing saidshift valve toward said low gear position thereof, whereby if saidsolenoid of said electro-hydraulic transducer means is inadvertentlydeenergized when the automatic transmission has the forward drive range,said acting means functions to hold said shift valve at said high gearposition thereof.
 8. An automatic transmission as claimed in claim 1,wherein said solenoid hydraulic pressure urges said shift valve towardsaid high gear position thereof, and said acting means includes springmeans for biasing said shift valve toward said low gear positionthereof.
 9. An automatic transmission as claimed in claim 1, whereinsaid holding means includes a manual valve manually operable to areverse drive position, said manual valve including means for generatinga reverse-select hydraulic pressure when said manual valve is placed atsaid reverse drive position, and means for supplying said reverse-selecthydraulic pressure to said shift valve.
 10. An automatic transmission asclaimed in claim 9, wherein said reverse-select hydraulic pressure urgessaid shift valve toward said low gear position.
 11. An automatictransmission as claimed in claim 10, wherein if said electro-hydraulictransducer means fails to operate normally when the automatictransmission has the forward drive range, said acting means functions toallow said shift valve to be held at said high gear position thereof.12. An automatic transmission as claimed in claim 11, wherein saidelectro-hydraulic transducer means includes a solenoid and is of the offdrain type that fails to generate said solenoid hydraulic pressure whensaid solenoid is deenergized normally.
 13. An automatic transmission asclaimed in claim 12, wherein said solenoid hydraulic pressure urges saidshift valve toward said low gear position, and said acting meansincludes spring means for biasing said shift valve toward said low gearposition, and means for generating a pilot hydraulic pressure acting onsaid shift valve to urge said shift valve toward said high gear positionagainst said spring means, whereby if said solenoid of saidelectro-hydraulic transducer means is inadvertently deenergized when theautomatic transmission has the forward drive range, said acting meansfunctions to hold said shift valve at said high gear position thereof.14. An automatic transmission as claimed in claim 12, wherein saidsolenoid hydraulic pressure urges said shift valve toward said low gearposition thereof, and said acting means includes spring means forbiasing said shift valve toward said high gear position thereof, wherebyif said solenoid of said electro-hydraulic transducer means isinadvertently deenergized when the automatic transmission has theforward drive range, said acting means functions to hold said shiftvalve at said high gear position.
 15. An automatic transmission asclaimed in claim 10, wherein said electro-hydraulic transducer meansincludes a solenoid and is of the on drain type that generates saidsolenoid hydraulic pressure when said solenoid is deenergized.
 16. Anautomatic transmission as claimed in claim 15, wherein said solenoidhydraulic pressure urges said shift valve toward said high gearposition, and said acting means includes spring means for biasing saidshift valve toward said low gear position, whereby if said solenoid ofsaid electro-hydraulic transducer means inadvertently deenergized whenthe automatic transmission has the forward drive range, said actingmeans functions to hold said shift valve at said high gear positionthereof.
 17. An automatic transmission as claimed in claim 10, whereinsaid solenoid hydraulic pressure urges said shift valve toward said highgear position thereof, and said acting means includes spring means forbiasing said shift valve toward said low gear position thereof.
 18. Anautomatic transmission for a motor vehicle with an engine, the automatictransmission having a forward drive range and a reverse drive range, theautomatic transmission comprising:a main gearing shiftable between aplurality of gear positions when the automatic transmission has theforward drive range and shiftable to a reverse gear position when theautomatic transmission has the reverse drive range; an auxiliary gearingincluding a planetary gear set driving connected to said main gearing, afirst frictional device and a second frictional device, said auxiliarygearing having a high gear position wherein said first frictional deviceis engaged and said second frictional device is disengaged and a lowgear position wherein said first frictional device is disengaged andsaid second frictional device is engaged, said auxiliary gearing beingshiftable between said high gear position and said low gear position;means for generating a servo activating hydraulic pressure; anelectro-hydraulic transducer means for generating a solenoid hydraulicpressure; a shift valve fluidly disposed between said servo activatinghydraulic pressure generating means, said first frictional device, andsaid second frictional device, said shift valve being shiftable inresponse to said solenoid hydraulic pressure signal to a high gearposition wherein said servo activating hydraulic pressure is supplied tosaid first frictional device and said second frictional device isdrained, and to a low gear position wherein said first frictional deviceis drained and said servo activating hydraulic pressure is supplied tosaid second frictional device; means for acting on said shift valve insuch a manner that, when said electro-hydraulic transducer meansnormally operate, said shift valve shifts to said high and low gearpositions selectively in accordance with a predetermined schedule whenthe automatic transmission has the forward drive range; and means forholding said shift valve at said low gear position thereof when theautomatic transmission has the reverse drive range.
 19. An automatictransmission for a motor vehicle with an engine, the automatictransmission having a forward drive range and a reverse drive range, theautomatic transmission comprising:a main gearing shiftable between aplurality of gear positions when the automatic transmission has theforward drive range and shiftable to a reverse gear position when theautomatic transmission has the reverse drive range, said main gearingincluding a plurality of planetary gear sets; an auxiliary gearingincluding a planetary gear set drivingly connected to said one of saidplurality of planetary gear sets of said main gearing, a firstfrictional device and a second frictional device, said auxiliary gearinghaving a high gear position wherein said first frictional device isengaged and said second frictional device is disengaged and a low gearposition wherein said first frictional device is disengaged and saidsecond frictional device is engaged, said auxiliary gearing beingshiftable between said high gear position and said low gear position;means for generating a servo activating hydraulic pressure; anelectro-hydraulic transducer means for generating a solenoid hydraulicpressure; a shift valve fluidly disposed between said servo activatinghydraulic pressure generating means, said first frictional device, andsaid second frictional device, said shift valve being shiftable inresponse to said solenoid hydraulic pressure signal to a high gearposition wherein said servo activating hydraulic pressure is supplied tosaid first frictional device and said second frictional device isdrained, and to a low gear position wherein said first frictional deviceis drained and said servo activating hydraulic pressure is supplied tosaid second frictional device; means for acting on said shift valve insuch a manner that, when said electro-hydraulic transducer meansnormally operate, said shift valve shifts to said high and low gearpositions selectively in accordance with a predetermined schedule whenthe automatic transmission has the forward drive range; and means forholding said shift valve at said low gear position thereof when theautomatic transmission has the reverse drive range.